From Worms Knowledge Base
The bunker can either be placed by a worm, or it can be an object that is already present when the game starts. The bunker is only slightly bigger than a barrel, and appears as a metallic hollow box on the landscape. Like other objects, the bunker falls down when the land below it is destroyed.
If the Bunker is placed by a worm, only worms of the same team may use it. They can enter and leave at will during their turn. Worms inside the bunker are protected by explosions from outside; the worms are effectively 'invulnerable' to enemy attacks. The Bunker itself, however, is not. Like a worm, the Bunker itself has limited armour, and blows up when that armour is depleted. Worms inside are slightly damaged in the explosion.
Worms inside the bunker should be given invulnerability, and the bunker itself, like a worm, should be programmed to lose 'health' as it is attacked. The game will be needed to keep an eye on how much health it has, and be ready to blow the bunker up when that health reaches zero.
This could have great potential in fort games. It may seem all too powerful to be able to walk into a virtually indestructible box after every turn, but you could also be walking into a trap. A well-placed girder, and the bunker is a prison, with the only method of escape being to blast yourself out, taking damage, or teleport, which takes a turn. Also, a nasty petrol bomb could turn things just as ugly.
A bunker is, when you think about it, pretty pointless. In Worms the idea behind darksiding is that you build your own bunker, using Blowtorch, Pneumatic Drill etc, and then guarding it yourself with mines and such. Adding such an easy-to-use alternative defeats this extra aspect to gameplay.