And what fortifications did the Seljuks and the Fatimids do to oppose them ?
I am trying to make a lecture for my university and I need information about Jerusalem, Acre and Damascus so I can make my own conclusions and observations. And I really can't find anything other than tourist sites.
There is a sizable amount of Crusader buildings in the Levant. The Holy Sepulchre was changed and added on to by the Crusader.
The Hospitaller church at Abu Ghosh is still there, including its twelfth-century paintings. There is a large amount of scholarship on this building. ( Kühnel, Fishhof)
The church of Saint Anne in Jerusalem, at the beginning of the via Dolorosa.
If you are interested in crusader churches, perhaps look into "Churches of the Crusader Kingdom" by Denys Pringle. Kühnel wrote multiple books and articles about the remaining crusader works.
Perhaps look into Jaroslav Folda's "Crusader Art."
I would be happy to send you my bibliography about the hospitaller church at Abu Ghosh (I wrote my MA thesis about this church)
The book The Medieval City Under Siege, ed. Corfis and Wolfe, has a chapter on castle and town fortifications in the Crusader kingdoms that would be perfect for you! It talks a little bit about what came before (town walls the Franks used as-is or added onto) and what was left standing after Muslims' reconquest.