My favorites are Defender (I and II) and Lair Defense/Lair Defense: Shrine.
Defender is a shooting game: enemies come in from the right and you shoot them or cast magic on them before they destroy your castle on the left. Defender II included an online competative mode which is also a lot of fun.
Lair Defense and Lair Defense: Shrine are tower defense games: the towers are all dragons, and you set them along a pre-defined route to keep the humans from stealing your eggs.
Both games offer a lot of upgrades and nearly endless gameplay. And best of all, they're both free!