Blocking the Mongols' passage south of the Yangtze were the twin fortress cities of Xiangyang and Fancheng. What resulted was one of the longest sieges the world had ever known, lasting from 1268 to 1273. For the first three years the Song defenders had been able to receive supplies and reinforcements by water, but in 1271 the Mongols set up a full blockade with a formidable navy of their own, isolating the two cities. This didn't prevent the Song from running the supply route anyway, and two men with the surname Zhang did exactly that. The Two Zhangs commanded a hundred paddle wheel boats, travelling by night under the light of lantern fire, but were discovered early on by a Mongol commander. When the Song fleet arrived near the cities, they found the Mongol fleet to have spread themselves out along the entire width of the Yangtze with "vessels spread out, filling the entire surface of the river, and there was no gap for them to enter." Another defensive measure the Mongols had taken was the construction of a chain, which stretched across the water. The two fleets engaged in combat and the Song opened fire with fire-lances, fire-bombs, and crossbows. A large number of men died trying to cut through chains, pull up stakes, and hurl bombs, while Song marines fought hand to hand using large axes, and according to the Mongol record, "on their ships they were up to the ankles in blood." With the rise of dawn, the Song vessels made it to the city walls and the citizens "leapt up a hundred times in joy." In 1273 the Mongols enlisted the expertise of two Muslim engineers, one from Persia and one from Syria, who helped in the construction of counterweight trebuchets. These new siege weapons had the capability of throwing larger missiles further than the previous traction trebuchets. One account records, "when the machinery went off the noise shook heaven and earth; every thing that missile hit was broken and destroyed." The fortress city of Xiangyang fell in 1273.