Not much info to go on there.
You might look at the terrain height where the character/camera are located versus what it is after you move and only allow movement if the Y increase is below a certain amount. This assumes that you have significant increase in height in your walls, mountains, etc.
Depending upon the size of the area, you could place a box or plane on each side of it, hide them and check for collision with them. You could make one large box to cover it all if it is a square or rectangular area.
You could also check the world coordinates and make sure that you do not enter a particular area. For a circular area, you could use vector3 to check distance.
Hope this helps. If not, please post more info / code.
LB
So many games to code......so little time.