I like it.
I suggest a fill bar or a percentage (like what you have for research) to show how long till the next monster comes. Also, it would be great to know what the maximum is number of each type of monster you're getting, like you've done with the catapults.
And since there is a limit based on sleeping space, maybe a way to destroy units or limit how many are produced (recruited? conscripted?), so you're not wasting your space on crappy lower levels monsters.
Also think about adding something else to do other than mine gold/research rooms for the player to do. Like if I'm mining gold, maybe randomly I find treasure, or if I'm researching, finding a way to upgrade one of the monsters, or a cool spell, or a way to boost how quickly I get monsters.