The UI seemed pretty intuitive. The only thing I didn't figure out at first was that I didn't see the empty slots showing how many perks you can have but I don't think that was an issue. I am used to playing RTS games though so clicking on the towers seems natural to me, maybe not so much for other players. The other thing that took me some time to figure out was that I need to stand next to the towers to restock them. I wasn't sure at first if that showed their hp or what.
I think I played on normal with the timeslowing character and got around wave 23, I was still doing okay but I didn't have more time to play. It seemed pretty easy at first but the difficulty did ramp up towards the end and I actually had to pay more attention. I was mostly just using the submachine gun and I only tried the other weapons on the turrets, I picked 1 of each just to see what they do. I haven't played enough to be able to say much about the balance of the weapons and upgrades. My first impression is that spending most of my money on my main character is better than spending it on the turrets and I only spent money on them when it was way cheaper than the alternatives. The fences didn't seem very strong but I only started using them late so it could be because of that. I didn't notice any enemies that felt overpowered. The homing missile guys seemed annoying at first but I don't think they actually hit me. The other projectile enemies weren't too hard to dodge either. I think my biggest worry was to not get surrounded by melee enemies.