Very nice idea! Having to strategically place enemies down in attempt to stop the hero is a cool idea!
I found it a bit hard to predict how the hero was going to move, since his pathing updated depending on enemy placement. It was also hard to remember how each enemy type moved, especially the two skeletons.
I think there is a lot of room here to a nice puzzle game, if it can become more predictable in what is going to happen. I often found myself placing things kind of randomly, and having it work out despite not putting much effort into it.
Having the board stay in the last state it was in would also help, since times I got close I couldn't remember where I had placed everything lol.
Overall a nice idea! Graphics and audio were nice! Lots of room here to expand!