Age of Defenders is a nicely executed, but very conventional, tower defense game, with two unusual aspects: It's both multiplatform and multiplayer. Well, two-player.
It can be played on a computer for free, in Flash; and there are clients for both iOS and Android. Given the size of the play area, my guess is it works better on a tablet than a smartphone, although you can swipe across the play area fairly easily. On the server side, the game is platform agnostic, meaning you could be playing against an opponent on any platform.
The fact that it's multiplayer adds a strategic aspect missing in soloplay tower defense games; you use the same resources to build offensive units as defensive towers, so you make a tradeoff between one or the other. Essentially, you stockpile units until you release them in an attack, when they try to run the gauntlet of the enemy's defenses and inflict damage on their base. Base hit points don't regenerate, but with two defense-minded players, the game can go on quite a long time.