The Tower is infinitely tall, as far as anyone can tell. Or maybe it's not even "tall," maybe it's just all that exists in this universe. Certainly no one's ever found the top or the bottom, and looking out from the Tower, all that is visible is sky and clouds, sunshine or starlight. The Tower varies in size and shape. Sometimes it's as narrow as a skyscraper, 100 yards across (I guess -- how wide is a city block?), sometimes it's a mile, two miles, maybe more. Sometimes it has a regular shape, circular or rectangular or other, and other times not.
The Tower has many environments. Forests, jungles, swamps, deserts, grassy plains, rocky wastes, even snowy tundra, all can be found on various levels -- often mostly open to the outside, but sometimes fully enclosed, and sometimes in between. Small lakes may spill over to the next level or lie serenely (or sullenly) in the midst of one. Of course, other levels are interiors, in any style imaginable that uses stone and brick and wood for building. Nothing we would consider modern, no levels built of steel and plastic and glass, but ranging from the finery of Versailles to the stone houses of the Anasazi Cliff Dwellers. Some levels are like basements and sewers and tunnels beneath a city, with fouled waters and refuse and crawling things. Others are like mountaintops with pure springs of water, or carefully tended gardens and penthouses above a city. But there is always another level above, another level below.
The Tower, of course, also has many inhabitants, with cultures as varied as the levels. Some of the inhabitants control multiple levels, others just a corner of one. The inhabitants are humans and near-humans, dwarves and giants, winged men and ape men and serpent men and... others. Some trade with others, some fight with others, some lead quiet, hidden lives, some wander from level to level while others spend generations reshaping a level into their own idealized home.
Who created the Tower? Is it truly infinite? What is its purpose? How did the inhabitants arrive? Is it the only Tower?