My cat pen has been a real asset! Most of the cats have little desire to go any further, at least until they see the neighborhood strays. They love to watch the birds.

My pen is about 9 feet by 30 feet and 6 feet high. It cost about $300 to build, including the shade cloth I put on part of it. The cat flap cost an additional $100 because it is made to insert in a doulble hung window. I used 2x4's, 1x3's, chicken wire and utility wire fencing---and a lot of screws. It wasn't hard to do if you can operate a drill type screwdriver. Of course you have to own the land it's on.