I really like the concept and the art style. The game looks really good and is extremely well polished. The only bug I found was that I summoned a demon in the same space as the portal, and couldn't actually control it as the game selected the portal as the demon was 'underneath' it.
However as an army building game, it is very tedious swapping between units and directing multiple demons in a direction, so a feature allowing for multiple selections at once would be a strong addition. Buffing the castle would also be recommended as you can technically play the entire game with only 2 demons needed as the big demon can 1v1 the castle. These two things combined left me a touch frustrated after spending longer gathering my army on their doorstep than the actual fight took.
But overall, I do like this and as a jam game it works pretty well and could definitely be expanded further into a fun game.