From the announcement:
The enemy has arrived at the gates! You are the last line of defense against the endless hordes of monsters that now line the outer walls of the citadel. Mount the walls and muster your courage. Bastion is on sale now!
Combining aspects of popular tower defense video games with the tactile feel of a cooperative board game, Bastion puts you in the middle of an all-out onslaught from beasts, archmages, dragons, and all manner of legendary creatures. The tension mounts every game as new foes arrive and slowly advance along the bastion’s walls.