Each stage you'll earn a number of gems that can be used to purchase items in between stages.
Your third instant special ability is yours to choose, by buying items in the in-game store. Both have a cool-down time (more so for the nuke ability) so you'll need to pick the best time to use them. You can deploy units on the ground to back up your knights or you have a 'nuking' ability for emergencies (like a flaming meteor). As an extra line of defense you also have three special abilities you can upgrade (that you'll unlock as you play) for when the towers are not quite enough. In between levels, you can upgrade various attributes of each tower to make them more powerful, fire faster, and decrease training time for deployable units.
