The herd of treekin followed Caelan, who made a left turn off the castle wall walk and down some stairs into the town proper. 4 assassins were skirting the backside of the exodus, herding the group along and gathering in stragglers. Caelan looked up and finally spotted one of the flagmen on a balcony above. The flagman gave him the green double flags and waved the horde to the end of the lane ahead.
Behind them at the doorway of the castle, two archers watched as the door was first pounded from inside and then attacked with axes and swords. Two more casks of thunder powder were back carried to them by two assassins. The three casks were placed in front of the double doors. They could hear angry, confused shouts and curses coming from within. A very large group of angry soldiers was trying to get out, and at them. Watching as the last of the refugees were climbing down the rough steps to freedom, the flagman on the wall waved double green at them. The wicks to the three casks were lit, and the four of them scrambled to aid the escapes as they fled. The flagman made a beeline to the breach and scrambled down to safety. His job was over, and he ran to the hilltop for all he was worth.
The detonation at the castle doorway ripped a hole in the side of the castle and through the troops inside. 100 were killed as the blast ripped a hole through their number, and several hundred more died as the castle wall fell in on them. Several hundred maddened dusty soldiers began hand-chain passing chunks of stone and wounded or dead soldiers. They stacked them in the hall and threw them down the stairs to the prison, since it was empty anyway. It took them 20 minutes to clear a way out, and another two soldiers perished as stones fell from the castle wall as they were clearing it. By that time, those running up the hill were almost there, with their back guard of stone and sand kin. The astounded soldiers that found the breach, stood gape mouthed looking at retreating slaves, through a shattered wall. There had been no orders from the commanders, so a Captain of the guard ordered them to pursue those escaping before their eyes. The soldiers were mad, and it was time to get some payback. 300 of them charged down the stairs and up the hill.
Bronn's group was at the first road juncture in the lane, and a flagman waved them onward to the last turn in the road ahead. That meant the flagman could see troops running to reinforce the castle as they came up the road he was watching. So they went on to the last turn. When the second explosion went off, they had rounded the turn into the last street, with two green flags from the signal man. Town people were peeking their heads out to see what was going on, but when they saw the mass of slave treekin running down the road, followed by very shady-looking characters, they ducked back into their homes. Seeing the damage caused by the strange raining stones, could wait until tomorrow. What none of them saw, or heard, was the signal man suddenly whistling and waving two red flags at the retreating mass of escapees. The flagman gave up and ran for safety.
When the first explosion went off, Arya's group ducked in a little more and waited for the resulting flood of reinforcement troops. But something lucky happened. From the house across the way, a door slammed open, and a man came out quickly, trying to dress and put on his gear. The man pulled out a whistle and blew four loud blasts. Recognition dawned on Arya's group as to just who the man was. His general epaulets flashed even in the dark. The man was in bed with a trader's wife when the explosions went off. He was still struggling to don clothing when Jon and Drena knocked him out and bound him, dragging him gagged into the alley. A hurried group of soldiers went past and then another. Their group, mixed treekin, Westeros, and other soldiers jumped out of hiding and made their way to the port. As the second explosion went off, they met about 20 more soldiers on the road coming hurriedly to reinforce the castle. A dozen archers were firing bows and crossbows at them when the soldiers realized they were under attack. They went down in short order, making almost no noise. Arya's group hit the port running and snuck aboard the first of the 4 ships in a row. The few men on deck were caught unawares and went down to arrows, and the team split, flowing over the decks of the four ships, clearing all enemies. Then the hard work of clearing the ships' insides without raising any alarm began.
On the hill outside of town, all the signalling was recorded, and the information was passed on to the leaders. The last signal came from the flagman watching the port that Arya's team had boarded the ships. As the 300 soldiers came up the hill, they could see the rear guard stop and divide into three groups that stepped apart from each other, 4 barrels rolled down the hill past the rear guard and towards the soldiers. Two soldiers stopped one of the barrels when it reached them, laughing, as their squad mates swarmed around them, and the barrels all went off. The explosions knocked almost all of them down, killing over a hundred, and many more were wounded. A heavy rain of arrows killed a hundred more. The rear guard finished off the rest and then slowly walked back up the hill.
On the wall, the captain who had ordered the attack put his hand out to stop more soldiers and reinforcements from going. Still, a newly arrived Lieutenant, wondering where the general was, shouted for another wave of men to attack.