Sentence:  Travelers entering from the desert were confounded by what must have <WSD>seemed</WSD> an illusion : a great garden filled with nightingales and roses , cut by canals and terraced promenades , studded with water tanks of turquoise tile in which were reflected the glistening blue curves of a hundred domes . 
Meaning of the word marked with <WSD> tags:  give a certain impression or have a certain outward aspect