I live in a similar situation to you. Before moving to the flat with a garden, my two year old cat stayed indoors all the time. Now I allow her out in the morning when I get up until I go to work. I trained her to come back by calling her name and rewarding her with 'Sweeties'(which I also call), which are small cat treat sweets. When I first let her out, every time she came in I rewarded with a sweet and I keep these in the same place in the kitchen drawer. I never leave her out when I go out and make sure she has plenty of toys left around when I go to work. When I come in, she can go out, but once it starts to get dark, she comes in. Other cats come around so the patio door always has a gap where she can run in. She does stand her ground, but if I hear cats growling etc I take her in straight away so she doesnt get hurt, or run near the road in fright. I know you cant watch them all the time and sometimes I feel like the over protective mother! If the cat has been spayed then other cats tend to leave them alone. Strangely enough the girl cats do not like other girl cats!