They besieged it for three years

Alamut, vestiges of an impregnable castle

Alamut, vestiges of an impregnable castle (photo Payampak source).

“they besieged it for three years, but they could not take it, so strong was it. And indeed if they had had food within it never would have been taken. But after being besieged those three years they ran short of victual, and were taken. The Old Man was put to death with all his men [and the Castle with its Garden of Paradise was levelled with the ground].” [1]
The Travels of Marco Polo on Alamut Castle