You can find many different types of cat flaps – some work with chips so that only your cat can get in and out. Others are manual, like ours. There are some that are built for thick walls. Some have flexible flaps, others have proper little doors.
Ours has a door that can be locked with the little yellow knob. During winter we have the door on because of the cold, but in summer the cats can come and go as they please. We have chew-proof cat net on the balcony.