A tower trap is very much like a dark room trap, only there are multiple layers for the mobs to spawn on. This type of trap has a much greater efficiency than a dark room trap though it is much harder to construct as it’s majoratively above ground level so will require a lot of building material.
Each layer in a tower trap should have two blocks of air in between, as all other monsters apart from zombies will not fall more than three blocks vertically. Also because of this you will have to interweave the layers of the tower so as no drop will be more than 3 blocks, this can be through strips of squares or whatever, the choice is yours.
The bottom floor of your tower trap should have some sort of canal transportation system to usher mobs toward the killing mechanism.