Cannonball Towers are the Squire knight's first kind of tower. Add the Knight to your "deck" (meaning equip him as one of your heroes.) Switch to him during a build phase, and hold
to bring up your towers. Press
to select the cannonball tower, and a to place it. Upgrading with
+
will increase the damage.
Use a lot of upgraded Cannonball traps at once to unlock this quickly.