Controls

Click or Space

Close the Professor's message and start the level

Click a slot ring

Open the build menu; click a tower to build it there

Click a built tower

Upgrade, Sell, or Move rally for a barracks (then click the road)

Click the ring by the entrance, or N

Call the next wave early for bonus credits

F

Fast-forward on or off

Esc

Close a menu, or pause

R

Retry after the run ends