My take on what this movie is about - SPOILERS
Here's my take:
The soldiers were being haunted either imagination by the deserters they were forced to shoot (hence the constant flashbacks), or the deserters really came back as ghosts to go after them. Due to the high number of deaths/burials in the tunnel, it attracts evil energy and hence the deads were able to come back alive there.
That's really nothing much to this movie.
Show me the holes!