Joan,

I don't have the invisible fencing, but I do have a doggie door. Our doggie door leads out to our enclosed patio. We have had 2 or 3 snakes on the patio, but none have ever come through our doggie door. Our doggie door fits into the concrete blocks and has two flaps. Our doggie door flaps are rather (not sure of the word I'm looking for) difficult to raise. So I don't think a snake would be able to push it open. I can't speak regarding any other critters. I could see an opposum or such animal being able to get in. Fortunately, they don't have access to our enclosed patio. Don't know if that helps or not.