When Siegecraft was first released on the App Store (by Crescent Moon Games, who have made some really amazing titles over the past few years), I remember it being marketed as "Angry Birds in 3D." And that description kind of works -- you are indeed flinging various weapons (though not of the aviary kind) at structures and trying to bring them down.
But I think a description like "active tower defense" works a little bit better. In Siegecraft, you are building towers, but instead of them just firing away on their own, you need to aim and fire them yourself, defending against invaders and trying to lay siege to enemy towns. The action is fast and fun, and there's a lot of it: You can play six campaigns with over twenty levels each, and three different races with their own abilities and towers. There's even turn-based multiplayer included, and if you play the game on iPad, there's extra content to see there as well.
Plus, Siegecraft is on sale right now, for exactly zero dollars. At that price, it's a must-play. That Angry Birds comparison might not have been all that apt, but there's a lot of fun here anyway.