Concept:

Make a game that qualifies as tower defense within the time period. 

In brief, this qualifies as a game where you place down static units to attack an onslaught of enemies. There are plenty of free online tower defense games to give you and idea, though being unconventional and innovative is encouraged.

Theme: TBA

Rules:

Standard rules apply. Make the game in the time limit, teams are fine, using existing assets is also fine for this jam.

Judging:

Judging is done openly, so make sure to vote when the jam period ends.