Here's a popular flash game inspired by the popular genre of Tower Defence, which is a custom game in Warcraft 3. Wave after wave of "creeps" pour out from various spawning points in increasingly more challanging forces and numbers. You have to build towers and upgrade them in order to keep the monsters at bay. If the monsters managed to get through your defences, you lose. The flash is pretty cool, but the map Element TD is much much better IMO.
Warcraft 3 (Element TD) based Flash Game:
Play it now
Inspired from the Warcraft 3 Tower Defence custom map Element TD.
If you enjoyed this game, you might also want to try out Flashcraft, which is also another tower defence game based on Warcraft 3.