Use the following trick to build a gate that you can use, but not your allies 
(to prevent them from using the resources around you). First, build a Fortress, 
Migdol Stronghold, Hillfort, or any kind of Tower. Enclose your city with walls. 
Have the Fortress or Tower be a part of the walls. When you want a unit to get 
from one side of the wall to another, have them garrison inside the Fortress or 
Tower. Then, set the gather point to the opposite side. When you eject the units, 
they will be on the other side of the wall. 
Note: This does not work on units that will not garrison inside buildings.