So I don't if this is the right subreddit for this, but I need help with one my cats.

So a buddy of mine asked my to basically adopt his cat because he signed an apartment lease with a buddy of his who is completely allergic. So I've had his cat since september and my apt smells like cat piss. I bought and enclosed litter box, cleaned it, shampoo the carpets and everything. I just came home and witnessed the new cat peeing on my wall! What kind of cat pees on a wall? She's peed on things before like my pedal board and winter coat but I always assumed it was because I had people over and she was scared. I opened the door and saw this female cat, with her rear to the wall, peeing. How do I keep her from peeing everywhere? My gf and I live together and she's not fond of cats but doesn't hate them. This is becoming an issue.

TL;DR How do I keep my new cat from peeing on my wall?

Edit: Thank you guys so much for the great advice! I'm going to be cleaning the box like crazy and I'll be calling the Vet looking into the other options. Thanks so much, you guys are the best.