It appears impossible to open from this side.

"fuck", Martin is stumped, "I'm going to go back to see if there is anything upstairs that we may have missed which may help us open this door", Martin says to Naru and Andrew, "Come with me if you want, if any of you stay here, we'll keep each other updated on the radio." Martin goes back to the beginning of the tunnel and looks up the ladder shaft with his assault rifle ready to make sure that it is clear and safe to climb...

Emilia appears to have accidentally or purposefully destroyed the top half of the ladder.

Naru passes Martin rope from her bag.

"Shit", Martin takes the rope from Naru and connects it to a grappling hook which he got from his pack and attempts to throw it up the shaft and pulls it to make sure that it is secure...

Naru looks around the room to get a better idea of where they are.

The grappling hook catches, but on the trapdoor lid, and Martin inadvertently closes it.

"Shit", Martin puts the rope and grappling hook back into his pack and goes back to the door, he paces the door for a bit and then pulls out the shopping list, he then reads it out loud to himself trying to make some sense of it...

The door rumbles open.
"About time one of you idiot guards checked in," came a voice from within.
"We almost thought you were dead."

Surprised that the door has opened Martin quickly raises his assault rifle and attempts to fire at the guards...

"What the hell?" yells the voice, which is then cut off in a burst of fire.
Weapons loading and the rustle of movement is heard.
"They are here already?"
"I thought they were still on the other side!"
"Yet again your plan falls short of success, Meka!"

Martin looks ahead to see what is in front of them...

It's a room with desks, radios and maps. It looks like the command bunker for the enemy forces.

Naru readies her weapons, ready to attack if necessary.

Andrew runs back to the corner of the room, into the shadows, and pulls out his Intervention. He sits there waiting to fire if necessary...

Martin goes into the room with his assault rifle ready to make sure that the entire room is clear of enemy forces...

He is fired upon immediately. Bullets smack into the wall beside him as foes to the left of the doorway open fire.

Martin instantly ducks and rolls to the closest desk and pushes it over to create some cover, he then attempts to fire at the guards with his assault rifle...

Naru blackens the room so the boys can use their thermal scopes.

Martin uses his night vision scope on his assault rifle to attempt to take out the rest of the guards...

The guards are firing continuously, even in the darkness. The desk is thick, but splinters and cracks under the barrage. They are firing too much for Martin to get a clear shot on them.

Andrew raises his Intervention and starts firing at the guards.

Martin quietly moves to the side next to one of the walls of the room staying crouched as he moves and attempts to use the night vision scope on his assault rifle to only make head shots on the guards...

There are about seven guards, all with heavy machine guns. The coordinated fire means that when one of them reloads, the others can fire. The hurricane of bullets is whithering as it tears into the walls around them, sending chunks of concrete cascading everywhere.
Andrew manages to take one of them out, but he alerts them to his location.

Martin attempts to make a head shot on another guard and then will quickly move to another position, staying crouched and attempt to make another head shot from there, constantly moving positions between shots, so that the guards can't work out where he is exactly in the darkness...