I've lived here 33 years and in that time I've had 3 stranger cats who would just come and go as they pleased. My own cats back then tolerated them - one (I still have) treated the last one as a great friend. The only time we had a problem was when a different one came in and sat unseen on the dining room chair - a lot of hissing from one of mine ensued and we finally discovered the intruder - he was black as coal so well hidden. I don't know who was more shocked me, him or the cat who discovered him.
You can get cat flaps which will only open via the micro chip but they have to push harder - neither of mine now would understand and I'd have to open the door for them 
Farage fails to report 5 million gift!
Changing from a Manual car to an Automatic after driving manual for around 50 yrs
