Gateways

Most Warzone map editors, such as FlaME, allow you to define gateways - they look a bit like this in the editor:

Your gateways should ideally be near choke points (narrow passages between mountains or other terrain obstacles) and be at least 4 tiles wide. AIs will build walls or hardpoint defences on the gateway tiles, except for the two tiles in the middle of the gateway where they'll either build gates or leave them empty so units can pass through.