This is widely believed to be the longest siege in military history.
Candia
Leningrad
Dien Bien Phu
Khartoum
Syracuse
Candia, capital of Crete. The city was besieged by the Ottoman Empire from 1648 until 1669 – twenty-one years of constant blockade. Crete at that time was a possession of the Venetians. On Sept. 27, 1669, the city surrendered after assurances the Venetians would be allowed to leave unharmed.