The Mamluks followed a conscious policy of not only slaughtering and enslaving the inhabits of the cities they conquered, but then destroying the walls and other architectural features. The point was to make the cities uninhabitable so it would be impossible for a new wave of crusaders to arrive from the West and re-establish a Frankish presence. The impact on trade was devastating.
Once the cities were abandoned, even the buildings that the Mamluks had not torn down fell to ruin. The fountains and aqueducts got clogged with debris and then crumbled away. The untended breakwaters crumbled; the quays cracked and broke up. The roofs collapsed; the winter rains found their way between the paving stones and then the weeds. Trees took root where once the busy markets had displayed all the riches of the orient. Travelers from the early 14th-century describe passing through ghost-towns already being buried in sand-dunes where once the great coastal cities of the Levant stood.
Meanwhile, trade moved elsewhere — for roughly two centuries to Famagusta — and more generally to Damietta, Alexandria and back to Constantinople. Trade also shifted to the Black Sea, and overland routes regained traffic.
Furthermore, the Pope imposed an embargo on all trade with the Islamic world between 1291 and 1344. Obviously, many merchants evaded or ignored the embargo, but the majority of merchants preferred not to risk excommunication and found new markets. As a result, the overall volume of trade between Europe and the Middle East was significantly reduced.
Eventually, squatters found the cities the Mamluks had tried to extinguish. Peasants put up a few rough walls in the corner of a former palace to make it habitable. No need for the great, hooded fireplaces, though, cooking could be done as before over a fire in the middle of the room. Traders passing through turned a former church into a tavern or a stables for their horses. Fishermen tied up at the broken quays, dumping dead fish where once the great dromons of the Byzantine Empire and the galleys of the Italian maritime powers had loaded luxury cargoes. Slowly, slowly life returned to the towns, and the ruins were increasingly cannibalized to build new structures more suited to the provincial backwaters these great cities had become.
But trade? The vast exchange of products, technology and ideas that had characterized the Frankish kingdoms of the Levant? It was gone.
Oh, some of the most precious products — ivory and opium, incense, spices and silk — still found their way to Europe, usually by more expensive routes that affected prices and so depressed demand. The most luxurious of goods could still find their way — just as slaves still moved in the other direction.
No, what suffered most was the trade in ideas. The Frankish kingdoms at the crossroads of civilizations had been a marketplace not just for commodities and luxury goods, but for technology and ideas as well. Once, tens of thousands of pilgrims who had annually made their way from West to East, exposing themselves to new cultures, new landscapes, new ways of doing things; now they all stayed home.
The economic impact of the crusader states is an important focus of my history The Holy Land in the Era of the Crusades: Kingdoms at the Crossroads of Civilization, which will be released next year by Pen & Sword.