Concept: creative, fitting approach to the theme.
Gameplay: I really like the drawing system it works very smoothly and is fun to interact with. I like the variety of units, their flavour texts are creative, made me smile. I think some extra text that describes their differences would be nice though. Also some info about the enemy units. And whatever the little potion icon and its number represented.
Another problem is that currently some units feel kinda useless, as just spamming zombies or vampires beats the game easily. Maybe if some enemies were vulnerable to only specific units, or maybe if only some units damaged the castle could vary it up a bit and encourage the player to use different unit compositions.
Other than that it was good, had fun trying out the units and trying to replicate the different drawings.
Art: very pretty spritework especially for the units. All looked nice and coherent. The beckground was maybe a bit too messy but that's just a nitpick.
Audio: the music was good, fitting for the vibe. I felt that some parts could use more sfx like the units fighting. The existing sfx are great though, gives good gamefeel when interacting with stuff.
Solid entry overall, it's well polished with only some smaller gameplay issues.