Tower defense on the go? Works for me!
The 3 worlds available have 2 map layouts each with both normal and hard difficulties for each map. This gives you essentially 12 levels to perfect. Each world offers different variations on the same core game. World 1 is your basic, core game. World 2 includes more characters and introduces crystals that can be placed to buff nearby units. World 3 has even more characters and more challenging maps.
DevPlay lets you see beginning strategies for each map which helps you think beyond your normally used strategy.
Your "towers" here are actually characters that would fit right into a final fantasy tactics game. Character units can be upgraded to do more damage and/or have a larger range of attack. Each unit does physical or magical damage and some enemies have resistances (or even immunities) to one or the other. The basic units consist of single-target physical melee, multi-target physical melee, single-target physical ranged, and multi-target magical ranged. Other units include your typical "slow enemies" caster, high damage single target Dragoon, and a Thief to increase gold received per enemy kill, among others.
All in all, this is no masterpiece. Not even close. But it's an enjoyable time waster and takes tower defense in a direction you'd expect from a hand held final fantasy game.