Polytopia is a bite-sized mobile game that distills Civ style gameplay into a 30 turn romp. It has randomized maps, tech trees, village placement, different unit types and unique civ based tile sets and unit graphics. It goes a long way to show how most 4X games are really just long form wastes of time. Mobile only though.
Dungeon of the Endless is of course exactly what the guys were asking about; a tower defense, round based, hero management, rogue-like dungeon crawler that is very tactical and complex at the higher difficulty levels. Works great on the iPad as well as on lower end laptops.