I like being able to place down units to fight and defend a dungeon. It's a shame you couldn't upload the latest version. I think there is a lot of potential for using different units to build different strategies. It would be great to see more synergy between the units and the dungeon to better support different strategies, and maybe some way to incentivise spreading out units to use the entire room. I think adding traps or unique room features could make unit selection and placement really interesting.
One thing I noticed is that the goal of the game, defeating the heroes early, prevents people from really exploring all parts of the game. You want to play with the units on the second floor, but you also want to wipe out the heroes before they reach it. I'm not sure if you planed to do more after the first group is defeated, but I think it would be nice if you had to defend against increasingly stronger groups as you keep defeating them. This would allow the player to experience all parts of the game, whilst still encouraging them to go all out with defending the dungeon. This could also allow people to revisit earlier rooms / floors and make some small upgrades to their initial floors.
I had some issues with spawning mobs: when trying to switch which mob I am placing, if the selection panel overlapped the dungeon floor I would end up accidentally spawning a mob at that location. It would also be nice if I could see some indicator of the mobs health while they are fighting.