What do you do when your pets do not like each other?

Right now I only have one problem cat. Lilly is the one who has become intolerant of the others. She lives in the main part of my flat, and Chelsie has my bedroom and bathroom, and the boys live in the vacant upper. Lilly will go after any other cat with screams and threats to drive them out, but as long as Chelsie stays on her side of the door she ignores her. That's as close as I've ever gotten to actual hostilities.