Empire Island

This game is a lot of fun, but it's really challenging.  It's a nice strategy game.  It has some elements of tower defense games to it, but includes the element of keeping a city together, with a population and an economy.

My frustrations with the game are that it requires coordinated use of the mouse, which makes it harder to play on the laptop, and that you only get 60 seconds between waves.  It's still fun.