Has anyone noticed WHEN that spirit appears? I have a theory it appears after some fixed time and it's HP is based on your level or how many spirits you have already killed.
It's hard but possible to get used to hotkeys (I miss W for traps instead of walls and 1 for gembombs). Anyway, I haven't used gem hotkeys yet, no numpad and I would need 3 arms (ofc you can do that on keyboard numbers too, but it's excruciating to remember all of them).
There are some bugs and some not very well GUI appearances. Like help box for walls/towers/traps/etc. covers your mana bar and those "achievements" cover part of the battlefield (it's not a big problem, yet...)
I have unlocked ~1/3rd of levels, but it will take more to get used to changes. And 169 (!) levels are just... epic playtime.