But bombs can not do U turn. for that reason they must have engine and that make them cruise missile not bomb more.
Just imagine entrance is north to south and your B-52 is coming from south to north.
you have only one option, cross the entrance take U turn then find good angle and then drop the bomb which that maneuver put your heavy B-52 at very dangerous position. Also what I am suggesting is that they provide only one corridor for invaders and concentrate most of defenses on that corridor. However I believe with use of terrain and mix of smart and complex plan of Tunnels like what
@VEVAK suggest is possible to save entrances.
--------------------------------------------------------------------------------------------------------------------------------
.