US Staff-Sergeant Joshua Danison cranes his neck to survey jagged ridges vertical and black above him on the eastern edge of Afghanistan, then reels off the rules here for survival as a Chinook transport helicopter thumps away into the darkness.

"Welcome to Combat Outpost Pirtle-King. Here we only move around at night. If you must move in daytime, make sure you stay close in against the northern walls, as most attacks come from there," he says. "If you must move in the open, do it at a run."

Nato commanders cite security gains, eleven years in the war, ahead of a 2014 withdrawal by most foreign combat troops, but there are still pockets like this, where the insurgent threat is so potent that US soldiers can barely move.

COP Pirtle-King, or PK, is a low collection of rockfill walls, trenches and camouflage net, built to help secure the sole road running through the strategic Kunar River Valley and intersect insurgent supply routes from Pakistan.

But the forested mountains on both sides provide perfect cover for the insurgents, including a persistent sniper whose aim has been steadily getting closer to the handful of US and Afghan troops here.

Faced with the threat of so-called plunging fire, soldiers have adjusted routines to carry out most tasks at night, apart from sporadic daytime patrols and manning a trio of guard towers where guns angle up to point high into the rocks above.

When not filling sandbags and extending their walls or doing vehicle maintenance in darkness, they sleep through the daytime heat or just read books and talk within the dusty walkways inside the walls, waiting to repel the next attack.

"PK is kinda like the childhood fortress that we never got when we were kids," quips Danison, 31, a race car fan from Concord in North Carolina, who spends his days following the fortunes of drivers half a world away.

"It is pretty interesting, the lifestyle is a lot different, being on kind of a vampire cycle, but it's pretty cool at the same time. We all enjoy it here," he says.

In a guard post along the northern wall, two bullet holes through the plywood remind soldiers here from Alpha Company of the 1-12 Infantry Regiment of the threat posed by a sniper they know as 'dushman', which is Dari for 'enemy'.

"It reminds us of 'douche'," says Sgt Rios Omar, 21, from Brawley California, using an American expletive. "There aren't many good snipers in this country, but this guy is good."

Written in biro beside the splintered holes is a defiant challenge: "f*** you, you missed me twice."

Use latrines at your own risk

Dushman shoots from somewhere on a green spur known as "the finger", above curved hills known as 'A Cup' and 'C Cup', but only vaguely similar to breasts. Sometimes fire comes from both sides of the valley, from the south and north.

"That kind of crossfire is usually a sign it's not Taliban, but more likely Hizb-i-Islami Gulbuddin. They're a bit more together," says Danison. "We have pushed them back into the hills though. They used to fire from pretty much right in front."

US troops in full body armor run across the central vehicle park and any open area to reach their rooms or shift between fortified positions, and use the exposed wooden latrines and showers at their own risk.

"If you have to go, we recommend you wait until night," Danison says. "Here at Pirtle-King, we're pretty much in a fishbowl, so we typically operate at night. It just mitigates any exposure during the day."

In a cluster of small rooms more like a submarine than a ground base, as many as 15 soldiers sleep in bunks stacked four high against a plywood wall marked outside by a target drawn where a Taliban rocket grenade hit but did not detonate.

"Bet you can't do it again," reads a sign spray-painted in black. A double-head axe on the wall is called the "Alamo Axe", in a dark-humoured reference to last ditch defense in the unlikely case the Taliban ever tried to overrun the post.

Pirtle-King, named after two soldiers killed at a smaller observation post near here in 2009, is one of a handful of bases here due to be shut down as U.S. troops withdraw from the area and handover to Afghan forces in the Kunar Valley.

Battalion Commander Lt-Col Scott Green says Kunar will make the transition successfully, as Afghan security forces were making strong improvements, including running the majority of patrols beyond the walls of Pirtle-King.

"We are moving security forces deeper into the valleys," Green says. "I know it's taking time and is not moving as fast as we would like, but we can do it here."