What an incredibly fun concept. I think you may have benefitted from the ability to move monsters from one spot to another, even after they've already been placed in one spot.
Otherwise super fun idea. I hope you decide to expand upon this idea even after the jam. Would be delighted to see more monsters and "placeable" items on the map. Maybe even have a Party of Heroes where each hero is stronger / weaker against specific things (maybe you can even give Inventories for the heroes so the player has to plan around what items the Hero holds e.g. a lockpick would mean the Hero can go through one locked door). Just thoughts that came to mind while playing.
Super fun and original idea. Nicely done, and nicely executed. Absolutely loved the art-style and sound design.