nice atmosphere & feedback.
I don’t get the altitude thing - the ‘fly’ perk should refill immediately after hitting an enemy. Don’t understand how you get past the first screen otherwise.

Being able to break the water fountain looks like a bug.

the water level should slowly pile up instead of raising from nowhere.

There are other ways to gain height, I probably should have detailed them more in the instructions considering how important climbing is..
Doing an up attack on a stunned enemy gives you a vertical boost. The intended strat there is doing two normal slashes to stun, then upslashing.
You can also double tap slash a stunned enemy to stick them on the wall where you can hang there and recharge your double jump.
Building up the bar is done by getting kills where the type of move used gives more bar. An easy way to get lots of bar is to downslash all the fountains for example

Yeah, I was also getting stuck pretty early on. Maybe I'm just bad at this, but I found that it was very easy to accidentally clear a band of altitude of all enemies, then fall below that band. In fact, after playing a couple times and getting a little bit better (but only a little) it actually felt like the game was punishing me: I was still having a hard time filling the bar, and I'd be on the ground with no enemies for a long way above me.

The techniques you describe sound fun, but I feel like there needs to be more of a ramp up to them.

Thank you very much for your feedback!
Yeah, the rising killfloor is to punish players that hang around the same height without ascending. I see that it definitely does so far too soon. Its a little tricky trying to add difficulty, I was trying to avoid the case where it doesnt get hard enough as you rise leading to you climbing endlessly until you're bored.