Like the idea!  As others have said, a sort of reverse tower defense.   I also like how it emphasizes strategy and potion management.    
Some sound effects during the fights would help a lot.   I also found the levels and the characters to be a bit samey.  
An interesting idea would be if the different warriors in your party had different skills/attacks that were more or less useful against different enemies AND you could set the attack order for your party.   This would give the player some interesting decisions to make.  For example, if the first two enemies were Frost Giants (weak to fire), but the final enemy was a Fire Demon (strong against fire),  the player might want to lead off with their Fire Mage followed by their Ice Mage.   So the Fire Mage takes care of the Frost Giants, dies at the Fire Demon, and the Ice Mage comes in and handles the Fire Demon.   Stuff like that.   Maybe there was something like that in the game already and I just missed it....

Sprites are cute too!  :)

Very interesting idea, it reminds me of something in-between a tower defense and a turn-based RPG, nice job! :)