Outdoor cat became indoor cat
One of my cats (pictured, Toulouse, initially one-year old) used to be an outdoor cat with his previous owner. She told me that he used to spend a lot of time in the trees, and was the only one of her several cats that would greet her visitors. He wasn't neutered at the time. When she gave him to me (having neutered him a few days earlier, as I have a female Persian that is unspayed), he was very afraid of me and of anything that moved. To make a proper introduction to my other cat, I kept him initially in my bedroom for the first week, leaving my other cat in the rest of the apartment (I eventually opened the door and they bonded just fine). In his first day, when cornered, he wasn't aggressive at all, but was almost completely unresponsive to food, water and play. He'd allow me to pet him, but his eyes looked annoyed (or perhaps afraid). 20 hours later, I gained his trust, as he would volunteer his head for petting, and ate the food I gave him. Two days later, he started reacting very strongly to play (i.e. chasing a toy mouse with a string). He became very attached to me, follows me everywhere, grooms me, meows when he wants to play, sleeps in my lap, etc., and now 6 months later, he is still acting the same way. He is very, very active, and it will normally take me 20-30 minutes of very intensive play to tire him every night before sleeping, otherwise he will stay up all night and wake me up.
What I find a bit puzzling is that he is still very, very afraid of any visitors, always running in a panic beneath my bed or inside a box. It's not that they're paying him any attention or trying to pet him, he just flees at the mere sight of anyone who isn't me. He is also still very afraid of everything outside my apartment, and won't step outside even if I put his favorite toys and food (though he'll stay firmly at the entrance). It's a one-bedroom apartment, so I would have thought that a previously-outdoor cat would find it too small. The only times that he has left the apartment were for his vet appointments (deworming, vaccines, blood test - he behaved surprisingly well with the vet, didn't require any restraints nor tried to escape). Yet, I think a part of him really wants to come out, as when he sees me leaving, he puts his paws on the door (standing on his hind legs) in the same way that he does when he wants me to open a door inside the apartment. He will even put his head outside the door for the first half-second, and then hurry back inside in a scared posture (there's nothing scary outside my apartment, just stairs, a very clean floor and an elevator).
So, it seems to me, he went from being a friendly, adventurous outdoor cat, to a people-phobic cat that can't stand being outside the apartment. He doesn't seem to have lost the energy he had when he was an outdoor cat, judging from our play sessions. I am not sure what can be done to make him less afraid of people and the world outside the apartment, I've tried with a cat tree (1.7m tall, which he loves), and giving him rewards after my visitors leave (he won't accept any play or treats while they're in). He has access to several windows that give him a very nice view of a plaza (I live in a third floor).
Any thoughts or suggestions?