“Bastions are formidable blockades that help to keep enemies out. When a Bastion is destroyed, Rubble appears and slows invaders that enter its area of effect.”
Bastion are a 3x4 wide fortifications with a "U" shape in the middle. This allows for the placement of 2x2 or smaller building (such as a sniper tower or anti-tank gun) within the gap in the middle.
When destroyed bastions leave behind a rubble that slows attacking troops within a radius. Place your bastions in an area where attacking troops will be forced to destroy them to maximize this effect.
The wall breacher (Engineer) does 25% less damage against a bastion than it does against a wall.