Robot Entertainment

Orcs Must Die! Unchained

The MOBA version of the famous "tower"-action defense.
This is currently in Beta and might change over time!

Age of Empires Online

Age of Empires Online (abbreviated AoEO), is a history-based RTS. The game is free-to-play—it is free for users to play, with the additional option of earning or purchasing premium content. Premium content may include access to many special items and blueprints, and more quests and features. Also, booster packs are now available, such as Defense of Crete and Skirmish. This former gives the player the ability to play in a survival type mode game, while the latter allows up to four humans versus up to four CPU opponents in a more traditional skirmish gamemode. (Soure: Wikipedia)

Orcs Must Die!

3rd-person action meets tower defense. Buy and place traps around corridors to stop waves of attackers, and when all else fails just finish'em off with your own weapons. Scores are made to be beaten, so feel free to revisit completed maps; all newly unlocked or upgraded gear will let you wreak even more havoc upon the not-so-uninvited greenskin horde.