hello nice concept , some little suggestion : 

• life bar for invocation and enemy
• possibility for our personnage to shoot at enemy
• an upgrade menu that appear between wave ( maybe juste one skill point per cleaned wave or enemy drop gold )
• tower upgrade ( health , regen , hurt back when hunted , an auto firing tower on second floor )

And a bug

if the first spell you use is mirror , it's use the card without doing anything