The game is interesting for sure, but the way I see it now, it's entirely RNG reliant. There's simply no way to kill the monster, until you get lucky enough that it doesn't get to your door and gets distracted by other rooms so your cannons can shoot at it freely.
I also like the upgrading mechanic but because of the aforementioned thing, it's entirely reliant on the monster's RNG whether it wants to bust in your door or not. And because it busting the door down means instant defeat, it's also entirely up to chance if you have well, a chance.
I realise this is only the first patch, so when updating the game do take this in mind. Decrease the RNG somehow, and also balance the game overall. There doesn't seem to be a reason to upgrade defence as like I said, once the door is down it's all over. The money is better spent on upgrading guns (if the monster allows), and winning is entirely up on whether the cannons kill it in time.
Basically the entire gameplay loop now is just to upgrade a bunch of guns, and hope for the best. So please, fix that. You've got something good going on here, and I don't want to see it go like this.