They saw it on the screen first, then the sound of rapid staccato fire drifted back to them over the still air of the city.
On the screen, many of those who'd been making their way up the motte had been mown down, but the walkers in the compound below gave voice, the moan amplified by more walkers joining them from the streets and houses surrounding the area of the docks.
Merryn grabbed the radio from her belt. "Sarge, we have a problem. Two groups of people are having a pitched battle to the east of here. One group has deployed a machine gun and every deadhead within miles has started to moan. I'd suggest you get your teams back to the boat. Grab what you can and be ready to leave."
The radio crackled. "Noted Scout 2. Teams, you heard the lady. Back to the boat with whatever you can easily grab. On the double."
A chorus of quick affirmations echoed across the radio waves and Merryn turned back to the screen.
"There's something going on to the north," said Tom, his voice hushed. "Look." At the top of the screen, a few walkers had staggered onto the grassy area surrounding the old fort. The man on the top of the wall had finished off the people trying to climb the hill, only a few managing to run away and escape the carnage. He had now turned his attention to the people inside and was carefully picking off rival gang members, turning the tide of the battle in their favour.
"He's not watching the hill," said Merryn.
"There are more walkers coming from the north." Tom peered at the screen. "Can you check out the street to the north of where they're coming from Merryn?"
Merryn grabbed the controller and moved the drone from where it had been hovering above the fort to head north.
"Oh hell," she whispered again.
The narrow street immediately north of the park was filled with the staggering forms of walkers. Hundreds were making their way towards the sounds of distant gunfire.
"What do we do?"
"We get the hell out of here, but first we need to try and warn those people. They may be scumbags but no one deserves to be overrun by walkers without being given a chance to fight."
Merryn steered the drone back towards the fight, increasing its velocity and lowering the altitude until it was skimming along at tree level, recording the advance of the walkers who had made it to the walls of the fort as it went. With delicate precision she guided the drone up past the head of the lone shooter who reared back in sudden alarm as she sent it rocketing upwards away from the fort so they had a panoramic view of the fort below.
Buzzing the machine gunner had had the desired effect as the man had noticed the walkers and was waving his arms and shouting. He changed his targeting to the horde of walkers below him and opened fire, mowing down the front rank of shambling horror. Others joined him on the wall and fired crossbows, dropped rocks, or shot pistols if they had them.
Hundreds more walkers were joining their fellows and a crowd of zombies now surrounded the ancient fortifications.
"They're doomed aren't they," said Tom, despair in his voice.
"Yes. We can't do any more for them now."
Merryn reached for her radio. "Sarge, you there?"
"Yes Scout 2, we're here. We've got a good amount of stuff despite the limited time we had. Two of the team even managed to find a full set of welding equipment, but to get it on the boat we'll need to start up the motor on the crane. If we do that, we risk attracting the attention of your walkers over there."
"Do it," replied Merryn. "Most walkers in the area seem to be heading into the fight at the old fort from what we can see from the drone. Get everything you can lift by hand onboard, leave the welding gear 'til last, then start the crane and get that on and be ready to cast off. I'm going to record as much as I can of what's going on here, then I'll fly the drone over to you so you can get it back to base with the information it carries in case we don't make it back to the boat."
"Roger that Scout 2. I'll give you a few minutes warning before we kick the crane into action."
"Thanks, Sarge. Talk to you again in a bit."
Merryn gave a low whistle, and the three scouts came back to join them in the centre of the roof.
"Report," she said.
"All quiet."
"Same here."
"The walkers were agitated," said Hope. "They started moaning when the gunfire kicked off and they're standing there looking east."
"You reckon we can get out without them noticing?" asked Merryn.
"We should be able to, but if there's more noise they may start moving around which makes it harder. Is it worth trying a different route back to the boat?"
"I reckon we're going to have to, as the crew back at the boat are going to start that crane up soon to finish loading. That's going to make some noise and the deadheads down there are going to be crowded up against the gate trying to get out."
"What options do we have?"
Andrew pointed to the corner of the building he'd been watching. "It's not brilliant if you don't like heights, but there is a ladder down over there. That takes you to another flat roof, quite a long one. There's a ladder at the far end of that roof. But I don't know where it goes."
"That'll have to do, we'll check it out with the drone before I send it back to the boat. Back to your stations for now please, then we're getting the hell out of here."
As the three moved back to their positions, Merryn and Tom turned their attention back to the screen.
A few remaining humans were clustered in a tight knot in the centre of the fort, surrounded by the cold grasping arms of the walkers. As they watched, the final few defenders were pulled under a sea of the undead and the fighting stopped. A few seconds later, the walkers stopped, turned, and moved the gatehouse of the fort.
"It's like watching a flock of birds. They moved at the same time."
"That's not good," muttered Tom. "Not good at all. Can we leave now, I'm feeling like this is not a good place to be."
"Agreed."
Merryn directed the drone back to their rooftop, then sent it scouting ahead over the route Andrew had suggested as a possible way out. She nodded and muttered "that'll work" to Tom, then directed the drone towards the boat.
As she did, her radio crackled into life. "Scout 2, we're starting the crane now, so we can load the final bits onto the boat. Time to get yourselves back here."
Tom grabbed the radio from her belt. "Noted Sarge. Merryn's directing the drone back to you. Please make sure it's stowed safely as there's some information on it Captain Summers will want to see."
"Roger that, thanks, Tom."
There was a distant rumble from the docks and the stuttering noise of an engine broke the silence. As soon as it did, the walkers from the compound below them gave voice, and Merryn grimaced.
Watching the screen, Merryn dropped the drone towards the boat, and smiled as one of the soldiers pointed at it. The Sergeant walked across the gangplank. Watched it land, gave a thumbs up to the camera, and switched it off.
"Right, let's get the hell out of here." Merryn threw the gear into her rucksack, signaled the others, and moved towards Andrew's corner of the building.
As Hope and Eddie joined them she spoke over the moaning chorus of walkers in the courtyard. "There's a way down here. We follow the roof to the far corner, then there's another drop into an alley below. Then we can head west to the river, then south again to rejoin the others at the boat." If that becomes a non-option, there's a floating platform tied up between us and the boat. We'll have to set it loose and ride downriver. Not a great option, but I do not want to get anywhere near the horde of deadheads that took out those gangs."
"I'll scout ahead," said Andrew and swung his leg over the ladder, spidering down to the roof below.
"I've got the rear," said Tom.
One by one, the team made their way down the long ladder to the next flat roof and joined Andrew at the next ladder point. He moved ahead again, dropping to the ground in the alley, and scouted ahead to the corner where the alley joined the wide concrete road that ran along the dock wall. He looked back, gave an all-clear, and turned back to watch the road ahead of them.
Once they were in the alley, Merryn whispered "okay, full speed ahead, let's get back on the boat before they have to leave us behind."
Five hundred yards to their south, the crane stuttered into silence and they became aware of moaning behind them. Nothing was visible on the docks, but the volume of noise was not just a few random walkers.
Merryn placed a steadying hand on Hope's shoulder. "Steady, they're behind us, not in front of us. We have time."
"I don't think we do," said Hope pointing. "They've managed to work out how to get out of the compound."