Lunacraft Wiki talk:Photos/Archive 1/@comment-4635610-20111220185054/@comment-4583259-20111222034609

It looks loke your entrances are only 2 or 3 blocks high. That will keep astronauts out. In this photo, there is a 3-block-high gap under all 4 barriers:



and it keeps the astronauts from going through. The only way they get from one pen to another is by climbing up the outer wall then jumping over the inner barriers.