I have 5 cats (2 males, 3 females, all spayed/neutered). They all used to be indoor, but because they urinated everywhere, especially one that slept with me, we now keep them outside. I really miss them. Is there anyway to keep them from peeing without an expensive investment?
Without fully knowing the situation, I would guess that this might actually be territorial spraying. Spraying isn't the same thing as pee (I'll spare the details). I hope someone has a better answer but I think chances are that with a mix of males and females, it is probably just too many cats for an indoor space.
To attempt to determine what the source of urine marking is, I recommend this ASCPA reference. There are a lot of possibilities and since this has a large scope I'm not sure it makes sense to repeat all that information here.