My 2 boy neutered cats are very good (if a little odd at times!) but they are crap at defending their territory.
At night they sleep in a basket in the hallway and only have the run of the hallway/landing. All other doors shut, including kitchen but food/drink left out. They can come and go as they please via the side door cat flap.
I have lost count of the amount of times I have gone downstairs in the night and caught sight of the chunky rear end of one or other of the damn neighbourhood cats exiting via the cat flap (my boys curled up in their sodding basket, one eye each open looking sheepish )
Sometimes it stinks in the hallway so I imagine the intruder cat is spraying, although I have never seen any physical evidence of spray/urine - I frequently see him doing so on the bushes out front.
Obvious solution would be to get a magnetic cat flap, but my boys don't wear collars. I do not want to lock them in at night either - they are used to and happy with their independence.
Would a Feliway spray/diffuser do anything? Or is there any other deterrent to stop all and sundry partying nocturnally in my hallway? Or do I just have to put up with it?