If you haven't yet experienced Plants vs. Zombies yet on one platform or another, we have one thing to say to you - where have you been? It's fantastic.
It's something of a hybrid of the castle defence and tower defence genres. In each level, the play area is a lawn split into several lanes - a bit like a swimming pool. Zombies approach along these lines, and if they reach your home base, it's game over. All you have to fight them off are plants. Some simply blast away at the undead, while others collect sun rays to produce coins, or act as mines or walls to slow the incoming waves.
In typical Popcap style, you're granted a sense of reward at almost every interval. New plants are introduced every other level, forcing you to re-think you strategies all the time. Plants vs. Zombies avoid the sense of stagnancy that affects so many tower defence games, and while it's not a pure TD game, it's the one we'd recommend above all others for all you Android owners out there.
One of the few must-have Android games, Plants vs. Zombies is probably the best "defence" game for the platform.