From Worms Knowledge Base
How would you like a girder that your worms, and only your worms, could walk through? The Security Door is just that.
Placed like any other girder, this variation has a slight tone of colour to it, namely the colour of your team. This indicates that worms on your team, and no others, can pass through it as if it didn't exist. For weapons, and other worms, it is a part of the terrain as usual.
Probably not as hard as it sounds to implement; recent third party programs for Worms have allowed players to take their worms on trips through the landscape as if it were thin air. By restricting this ability to a certain team, and to a certain portion of the landscape, this girder can be used a team-specific door.
This tool has huge potential. It could make fort games more interesting; darksiders' tunnels can be blocked off to intruders; and territories established by blocking off areas of the map to opponents, but not restricting yourself.
It gives a greater sense of posession seeing a girder with your team colour, knowing that it blocks the enemy but gives way to you. Nobody likes it when they place a girder and find it turns later, being used by the enemy to their advantage. This would be a very useful tool indeed. The idea behind it isn't the most realistic of ideas....but then neither is Worms.