Post-LD edition: This version of the game contains changes made after the official end of Ludum Dare.
Towers do all the work in TD games. But who will defend the defenders themselves?TLDR: Drag-and-drop to build towers, click on them to enter an inner TD game. If you kill most enemies in the inner game, the tower will level up.
The goal is to prevent the enemies from reaching the end. If too many enemies do, you lose. You do this by building towers via the drag-and-drop menu on the left. Each tower you build starts at level 1. The yellow bar under the tower shows progress towards the next level.
But the yellow bar will not fill by itself! Click on the built tower to go inside. There is another tower defence game! Every time an enemy reaches the end, the yellow bar shrinks. If you kill enough enemies in the inner world, the corresponding tower will upgrade.