A tower is any vertical structure consisting of multiple levels of a similar shape stacked on top of each other, usually connected by stairs. Towers require abundant human populations (low savagery, large tracts of neutral land) and a high number of secrets to be generated in world generation, as necromancers cannot be elf or goblin.
In Adventure Mode, necromancers are most commonly found in towers (I). The towers generally have a few necromancers and a lot of zombies. Becoming a necromancer yourself requires reading about the secrets of life and death, which are either found on a slab or in a book. If you decide to attack the tower, you might want to become a night creature first by drinking a vampire's blood or being bitten by a werebeast. Zombies and other undead will not bother you if you yourself are undead. Once you learn the secrets of necromancy, all remaining undead in the tower will no longer be hostile toward you, so simply making a mad dash for the slab may also be a viable tactic.
"Tower" listed as a neighbor on the embark screen indicates the presence of a necromancer tower, similar to adventure mode. The necromancer will regularly send undead to attack you.
As a player construction, a large tower serves the same role as a below-ground fortress, but without the drawbacks of cave adaptation: all of its tiles being Light Above Ground, so the dwarves will always be exposed to sunlight. However, it takes a large amount of resources (usually stone and wood) to construct a tower; while a source of clay or green glass can alleviate this, it is still much less efficient than carving out a fortress entirely beneath the earth.
Usually, a tower is built from constructed walls, floors, and stairs, although on some levels (such as those being used for the barracks) walls may be replaced by fortifications. With sufficient planning, a tower can also be cast from obsidian.