Most sources seem to suggest there were humans, but no fire. Were humans and titans on the same plane of existence? Did monsters roam the world? Which titans and gods existed at this point?
Who was in power in those times in each realm? Was the Underworld a place you could physically get to?
There is not a consensus, since there are so many variant traditions of these old myths, but a tendency toward a certain "main stream" of narrative and chronology is identifiable, much of it relying on Hesiod's texts—the Theogony and Works and Days—as a base, since he is considered, since ancient times, to be one of the earliest of all the mythographers.
The degree to which a myth might be described as "mainstream" or rather more obscure is the degree to which it adheres to or deviates from Hesiod's narrative. Some of the details supplied even in his own material, however, contradict one another.
Indeed, as you point out, many sources, Hesiod included, envision humanity as existing during the Titanomachy. Chronologically, the story about humans being deprived of fire takes place after the end of the conflict between Kronos [Cronus] and Zeus, but at any rate, going by the Roman writer Hyginus, in his Poetica Astronomica, human beings did have possession of fire before Prometheus stole it on their behalf.
It was on account of a trick that Prometheus had played on Zeus about the division of sacrificial oxen between the gods and humankind that, in indignation, the king of the gods took fire away from humans. Prometheus was then punished for giving it back to them. Moreover, this rendition of humans does not seem to have existed at the time of the war between Zeus and his father, since these were created in a collaboration project between the Olympian deities and the Titan brothers Prometheus and Epimetheus after the end of the hostilities with Kronos.
Really Good Times
The Works and Days and several later sources refer to the rule of Kronos over the world as the Golden Age, in what is referred to as the myth of the Five Ages of Man, with the Golden being the first and best one. We live (or at least Hesiod believed that he, circa 700s BC, dwelt) in the Iron Age, the last and worst of the five.
Kronos' defeat marked the end of the Golden Age and the beginning of the Silver, which was eventually followed by the Age of Bronze. Between that and the Iron Age was the Age of Heroes, where most of the action in Greek mythology happens, such as the life of Herakles [Hercules], the voyage of the Argonauts, and the Trojan War.
Going by the chronology, the people created by Prometheus should represent the beginning of the Age of Heroes. The myth of the Flood of Prometheus' son Deukalion [Deucalion], complicates this, though, since Zeus supposedly used said flood to wipe these humans—the ones made by Prometheus—off the face of the earth. This was merely one generation after the Olympians had constructed the first woman Pandora, who would thus become the mother of modern humanity.
Rulership, Cities, and Living Conditions
Bringing the focus back to the time of the Titanomachy, according to the Works and Days, "the deathless gods who dwell on Olympos [Olympus]" made the "Golden race" of men "in the time of Kronos when he was reigning in heaven." These people were always partying, were never sick or unhappy, never had to work hard, and never grew tired or old. They eventually died but it was always in their sleep. The uncultivated earth freely produced food for them and they were "rich in flocks and loved by the blessed gods."
According to Nonnus' Dionysiaca 41, the oldest city in the world was built by Kronos, and this was Berytos (at present day Beirut, capital of Lebanon), which was named in honour of Kronos' niece the Oceanid Beroe. The first of the Golden men came to live in Berytos at the same time that Kronos' son Zeus was born, which would imply that Zeus was not involved in the creation of this first race.
This story that Nonnus tells, however, is explicitly set against a different version of the myth which he narrates, in the same book, regarding the foundation of Berytos. In this second account it is claimed that the city was established thousands of years later on, as the personification of a different Beroe, who was the daughter of the goddess Aphrodite by the Assyrian prince Adonis.
According to Pausanias, at Olympia, the original site of the Olympic Games in Elis, a temple to Kronos was built by the Golden men. In that era, according to Plato's Laws, the world was also inhabited by Daimones [Daemons], "Spirits," which in Plato's time meant a sort of divine being who was higher than humankind but lower than the gods, and who was thus able to engage in mortal affairs in a way that the gods were too transcendent to do.
These Daimones were appointed by Kronos as kings and rulers for the cities of mortals, essentially because they were "better" than lowly humans in the same way that humans are supposed to be on a "more divine" elevation of being than cattle and goats. Thus in that Age there was "peace and modesty and orderliness and justice without stint," which "made the tribes of men happy and free from feud."
In The Statesman, Plato says that during Kronos's reign the concept of nations or countries did not exist, "nor did men possess wives or children... or families". They had fruit in abundance collected from trees and other plants,
And they lived for the most part in the open air, without clothing or bedding; for the climate was tempered for their comfort, and the abundant grass that grew up out of the earth furnished them soft couches.
Iambi fragments from the poet Callimachus seem to describe the notion that during the Golden Age animals could speak but that humans could not. Eventually Zeus reversed this scenario, which engendered a large part of the difficulties in relations between man and beast, and presumably it is the men of the Silver Age who were the first human beings to possess the power of speech.
The Origin of the Armour of Athena and Zeus
A small number of monsters occurs in stories set during the Titanomachy. Strictly speaking I would say that only one of them is described as "roaming the world." According to Diodorus Siculus,
Gaia, the Earth, had given birth to "a certain frightful monster" named Aigis [Aegis], which "breathed forth terrible flames of fire from its mouth". Emerging first in Phrygia, the monster burned a path of destruction throughout the entire region, across the Taurus Mountains and all the way to India, turning back around to scorch a swath of flames via Mt Lebanon, through Egypt and on into Libya...
~ From my Answer to another Question
The goddess Athena, to protect her homeland, for she had been born and raised in Libya, killed the monster and wore its hide as armour, which, from the creature's name, was also called the Aigis.
Hyginus' Poetica Astronomica offers a rather different version of the myth, in which Helios, the Sun-Titan, had a daughter named Aix [Aex], who terrified the Titans since she had an exceedingly beautiful body but "a most horrible face." After the Titans begged Gaia to hide Aix from them, the earth-goddess concealed Aix "in a cave in the island of Crete", where she later became a nurse to the infant Zeus. Hyginus seems to go on to describe Aix as a sort of Gorgon and to imply that Zeus, after he had grown up, wore her skin as armour, which from her name was thus called the Aigis.
The Serpent-Bull
Ovid, another Roman writer, says in his Fasti that Gaia had given birth to a shocking monster in the shape of a bull that, in place of hindquarters, was composed of a snake. Whoever burned the intestines of this creature "was destined to defeat the eternal gods." A gigantic ally of the Titans killed this monster serpent-bull and almost achieved this end, but the kite-bird snatched the innards and delivered them to Zeus, and was rewarded for the feat by being made into part of the constellation Boötes.
Ekhidna and Her Son
In Dionysiaca 18, Nonnus describes a humongous "dragon-footed" creature he calls simply "Ekhidna's [Echidna's] son", who spat venom into the Sky in such profusion that it created large clouds of poison, and who was so huge that the birds flying through the air would get caught in the vast mass of his dreadlocked hair, which hair he would then use to sweep these birds into his mouth to chew them live for dinner. This creature, apparently an ally of Kronos, was slain by the war-god Ares.
Ekhidna's claim to fame is that she is the mother of about half of all the major monsters of Greek mythology. She never otherwise features in the Titanomachy but the implication that she had a son who was on Kronos' team during the Titans' War is that she was somewhere out there in the world in that era. Going by Theogony 295-305, she was ageless and immortal; part goddess, part nymph and part snake; subsisting on raw flesh, and dwelling in "a glorious house" granted to her by the gods in "a cave deep down under a hollow rock far from the deathless gods and mortal men" somewhere in a place called Arima.
Briareos-Aigaion
In now-lost Titanomachy stories (e.g. from the epic by Eumelus of Corinth), the Hundred-Handed Giant Briareos, whose human name Homer tells us, in Iliad 1, is Aigaion [Aegaeon], is a monstrous ally of the Titans who has been born of the Sea. It is he who in the Fasti almost achieves the defeat of the gods by nearly feeding the serpent-bull monster's guts to the flames. In the Theogony and the Iliad, however, he is very much part of the camp of Zeus and the other Olympians.
The Jailoress of the Abyss
Kampe [Campe], who was perhaps the largest monster in the universe, was a horrific winged, caterpillar-like creature with a scorpion-sting, and numerous heads and bodies, who had been posted by Kronos to guard the entrance to Tartaros [Tartarus], the vast storm-wracked pit of the Underworld in which the Titan had imprisoned his own brothers the triplet Cyclopes and the triplet Hundred-Handed Giants. Towards the end of the Titanomachy, Zeus killed Kampe and freed these six uncles of his, and they helped him to win the war.
River-Gods, Oceanids, and Winged Titans
At least three generations of Titans and pretty much every one of the major deities in the pantheon had been born by the time of the Titanomachy. Regarding the Titans, we know this because the first ever recruits in the war were the four children of the winged Titan Pallas, namely Nike, the Goddess of Victory, and her siblings Zelos (Zeal), Kratos (Power) and Bia (Violence). Pallas was himself a second-generation Titan, being the son of Kreios [Crius], one of the brothers of Kronos.
Certain of the children of the eldest Titan Okeanos [Oceanus] had definitely been born. One of these was Pallas's wife, the Underworld river Styx, who was also the mother of his children. Styx's brother the river Akheron [Acheron] gave aid to the Titans and was afterwards cast into the Underworld for this by Zeus. Their sister Beroe is the Oceanid after whom Kronos named the city of Berytos. Another Oceanid, Metis, is the one who helped Zeus to trick Kronos into vomiting the children whom he had swallowed at their births. She became Athena's mother.
Hermes, Dionysos, Zagreus, and Pan
The only one of the Twelve Olympians that I surmise to have been born after the end of the war is Hermes, since his birth tale in the Homeric Hymn to Hermes seems very much to be set in a time after Zeus has been established as the undisputed ruler of the universe.
In his Commentary on Virgil's Aeneid, however, Servius does place Hermes in a story about the wedding of Zeus and Hera, which by the commonest accounts should have occurred before or during the Titans' War, since the sons of Zeus and Hera are participants in the fight. (There are, though, somewhat esoteric allusions made by Nonnus and Callimachus to the idea either that this wedding day lasted for three hundred years, or that Zeus had to wait three centuries before he could marry Hera.)
Two other major gods who were undoubtedly born millennia later are Dionysos [Dionysus] and Pan, both of whom bear several generations of earthbound mortal ancestry on their mothers' sides, with Pan typically occurring as a son of Hermes.
In Orphic mythology Dionysos is a reincarnation of Zagreus, an ancient son of Zeus born to his own daughter Kore before she became Persephone, the wife of Hades, the King of the Underworld. The tales of Zagreus contain a completely different version of the Titanomachy in which humans are created from mud mixed with the ashes of the Titans slain by Zeus for having chopped Zagreus to pieces.
As far as I'm aware, we are nowhere supplied in the ancient sources with even a loose breakdown of the political structure of the universe before the ascendancy of Zeus and his team beyond a couple of references to Kronos "reigning in heaven" or being "the first king of heaven". According to Apollonius Rhodius' Argonautica and Lycophron's Alexandra, after their release from their mother's womb, Kronos and Rhea attacked the preceding rulers of the world, two other Titans named Ophion and Eurynome, whom they wrestled and defeated, thereafter casting them into Tartaros or the Okeanos River.
If Ophion is supposed to be a cypher for Okeanos, then it seems that after Ouranos [Uranus], he was the first ruler of the universe. Presumably all the realms in which we see continuity, in terms of the presence of the dominant deities straight from primordial times through to the reign of Zeus, were ruled over by these gods during the Titanomachy.
So Okeanos would have been the lord of the Okeanos River, which was his own body, which encircled the entire earth; and Pontos [Pontus] and Thalassa, the male and female personifications of the Mediterranean Sea, were probably envisioned as governing their own body/bodies.
Erebos [Erebus], the personification of the darkness of the Underworld, probably ruled the nether realm, perhaps together with his consort Nyx, the Goddess of Night; and Aither [Aether], who personified the bright upper air between the clouds and the dome of the Sky, probably reigned over the region which was his own body, maybe together with his consort Hemera (Daytime).
In Orphic cosmology the passage of time and the cycle of the seasons is caused by the friction of the two cosmic serpents Khronos [Chronus], the God of Time, and his consort Ananke (Inevitability), slithering against each other to cause the Sky-dome, around which they are wrapped, to rotate on an axis so that the constellations move across the heavens as the year (literally) rolls around. Thus this realm of Time would have been ruled over by these two primaeval divinities.
In Theogony 421-427 statements are made to the effect that Hekate, the goddess of witchcraft, held some special position of privilege "on earth, in heaven and in the sea", that this was a prerogative she possessed even during the reign of Kronos, and that Zeus respected this by allowing her to maintain her authority upon his accession to the throne. In Lines 411-412, "Zeus Kronides [Kronos' Son] honoured her above all."
The Netherworld
And yes, the Underworld was a conceived of a physical space. The body of Styx, who was the mother of Zeus's first recruits, flowed from a mountain in Arkadia [Arcadia] down into this realm, where, as we have seen, Kronos had incarcerated six of his brothers, guarded by a humongous monster nymph; and Zeus descended into this realm in order to free them.
Parts of the above have been adapted from my participation in a post about the Titanomachy on MythForum.com
