Creeper World is a really nice Defense-RTS game, imo it ‘s really worth a try..
Play Creeper World (demo)
others nice game to check (if you want) too..
KillMe (Karoshi in a different flavour)
Fortress Guardian 2 (mediameg always on the breach)
SteamBirds (Boardgame ftw)
Final Ninja (Nitrome’s Pixelations)