http://play0ad.com, 0 A.D, a free, open-source, cross-platform real-time strategy game. GPL 2+ license for the game engine and the CC-BY-SA for the game art.
Tower? heck take a group of villagers with your army. Build castles in enemy territory as you capture it! (or maybe bombard towers) That doesn't mean the game is broken though.
Picked up Age of Mythology: Extended Edition during the Steam Summer Sale. My friends and I used to play a ton against each other over LAN when we were kids. Now we're all in different states, but multiplayer works really well so that's not a problem!