A Postern gate in the walls of Helm's Deep
A Postern gate is an opening in a wall through which only allied infantry, cavalry, and heroes may pass. They universally cost 300 resources, and can be installed at Wall Hub walls, the walls of Minas Tirith, and the walls of Helm's Deep.
This type of structure alone cannot be seen by enemy units, and can only be destroyed if its wall-segment is destroyed.