The peoples of the islands that surround this tower want some of that knowledge and power. They will pay nearly anything to have folks who hold their lives in less regard delve into the unknown depths of this tower and retrieve the ancient artifacts and knowledge of the long-dead empire.
And you really must have almost no regard for your life and for your safety to enter the tower. In the centuries and millennia since it fell, it has become overrun with monsters and creatures of all sorts. The upper levels, those that force themselves into the void of heaven, are filled with all sorts of alien creatures, such as angels. The lower levels, those that are beneath the waves, have become overrun with sea devils and other oceanic monsters. These creatures, along with those constructs and beasts left over by the ancient civilization, mutated or eroded by the forces brought into the structure, guard the secrets of the ancient peoples.
Essentially, this is a megadungeon concept that becomes more dangerous the higher up and the lower down you go, rather than just in one direction. Some specific factions, items, and monsters that might be found in Babel, The Tower Of Spirits are detailed below.
Normal DnD Creatures in Babel
The rats here are like fish. Or they are like light. Mundane creatures look more normal the closer to the center of the tower they are, but the lower down or the higher up you go, you will find that they have changed to suit the environment. In the lowest levels, the rats look like quadrupedal fish with unblinking eyes and toothy maws. In the highest levels, the rats look like balls of light with four spindly legs. Mostly, you will just use the same statistics, but switch out appearances.
The zombies that are low down are bloated with water and the skeletons are covered with kelp and sea stars. Vampires have eyes of black and mouths like lampreys and can become goblin sharks or clouds of hagfish slime. In the upper levels, zombies are the corpses of those who have died, possessed by space ghosts or alien energies. They do not decay because of the alien effect on their flesh. Those upper levels have additional shadows even in places where light shines. Vampires of these upper levels are featureless and smooth, until they open their mouth and you see a hole ringed with teeth of glass.
Cephalopoid mind flayers hide in the darker corners of the lower levels, and eye-covered beholders float through the halls of the upper levels.
Not Normal DnD Creatures in Babel
Angels live in the higher levels. They are made of rings of gold and balls of fire. They are covered in circles of eyes and limbs of animals. They speak only in the language that the stars throw at each other in the voids between them. These are spirits of order whose goals are harmful to meaty creatures such as humans, for flesh is chaotic and disorderly. They can take forms of meat, but only do so begrudgingly, and they remove creatures from their prisons of meat to bring them in contact with the greater order of the universe. They fall into orders, levels, and circles, and each celestial being has a role and a place. (angels will be described more in-depth in another post)
Saratan, the great crab, controls one of the lower levels. They are the size of an island, and on their back there lives a culture of piscine peoples.
Alien fungi and extraterrestrial flora grow on the higher levels. Those who breathe in the spores and the pollen contract unknown diseases and alien allergies, and some might become the mold-controlled puppets called Spore Cosmonauts.
Some fish and other ocean animals have adapted themselves for the slightly less wet corridors and rooms of the lower levels. These are angler fish that crawl around on the ceilings and floors of the tower and octopi that bound along, hunting anything they near.
Factions of The Islands
A cult that worships the fish gods Father Dagon and Mother Hydra want artifacts and creatures from the lower levels. Some even say that their gods dwell in the deepest parts of the tower, and they will pay ludicrously to bring some of them to these piscine deities.
A church of Bog wishes for more information about the kingdoms of heaven and about the nature of Bog. If you retrieve information about either of those things from a bonafide angel, they will pay you handsomely. They would also totally be down to get some holy relics to treasure or unholy false idols to destroy.
A slave ship has arrived recently. The people here consider slavery horrible, for it infringes upon the freedoms of the people, but they don't consider the fish folk people, so they're completely fine if you kidnap some fish people from the tower and sell them to the slavers. The slavers will pay additional money for aquatics, as they can work in underwater environments.
There is a wizard that requires the ichors and the parts of angels for spells that they are working on. They will reward with not just silver, but also with the spells and magical items themselves, if you promise not to share them too much.
The tribe on a small island near the tower considers it to be the home of their god. They hate their god. If you can bring back the head of their god, they will reward you with fame and honor, as well as a high standing in the tribe.