The classic Mages tower with libraries, artifacts, special spots for magic rituals or magic practice, was hoping for a spiral staircase but i don't think that's feasible unless it's two squares wide. bed rooms. Dorm hall. Vanities, Statues .A cellar with giant crystal and a roof with a portal! :P :)
Other tower option could be the classic Dark Tower with everything dark and a bit scary. Glowing purple or red lights. Lava pit in the center. An organ near the top and at the top a council of darkness with multiple chairs.
May not be the best idea but it's something. I just like towers to put in a game. They have a special presence being a tower.