Very nice concept!
I had a lot of fun thinking about the most efficient placement I could do. I also really like the looming menace of the incoming monster. I wish there was a marker arrow on screen showing where and how far the monster is.
One thing I noticed was that my heroes didn't react to getting hit by the Leviathan. There was a path between them but I don't think they reacted to the damage. Having them engage (or retreat if there is no pathway) on taking damage would be nice.
Well done!