This trench has been around for a while. The cut birch saplings support leafy branches that work as camouflage. Those had to be refreshed at regular intervals as the leaves dried and fell off. When troops were dug in for static defense, they had to stay alert 24/7. In daytime, the biggest danger was enemy snipers. Anyone careless enough to expose his head for more than a couple seconds risked having a bullet put through it. During the nights, guard posts had to stay alert for any enemy infiltration attempt. The enemy might want to mount a night-time raid, an all-out attack, or just trying to catch an unwary soldier for interrogation.
Snipers were a danger in the dark, too. If someone was stupid enough to smoke a cigarette while looking for enemy infiltrators, the glow from the cig would surely draw a snipers bullet. Just aim a little above the glow, and – bam! – smoking kills. Guard posts had their personal weapon plus some hand grenades readied and close at hand. A string to a bell or can in the nearby bunker would summon an officer or the rest of the squad. The barbed wire entanglements in front of the trench often had cans with a few pebbles in them hanging from the wire. If someone cut the wire, the cans would rattle, alerting the guard post that something was about to happen. A flare gun would illuminate the area, exposing any enemy trying to creep up on the position.
If the enemy made it into the trench, swift action was required in order to oust them. One didn’t want to be inside a bunker if someone lobbed a grenade into it. Submachineguns were the weapons of choice for both attackers and defenders; the weapon had been developed for trench warfare during WW1, and had been improved in time for WW2. Rifles had too small magazine capacity, and machineguns were too unwieldy in the narrow confines of a trench. Knives were useful while infiltrating and if it came to hand-to-hand combat. Trench raids and assaults tended to be brutal affairs, which was all the more reason to keep the enemy out of the trench.