Patsy, you're thinking of Erengrad, the Kislevite port, which was indeed besieged and then assailed by troops from land and sea, as Marauders came from the waters and Hellcannons battered down the walls. Those events took place in the recent past, during the Storm of Chaos.
The city of Praag came to fame for falling (after a long siege) during the Great War Against Chaos, two hundred years (and change) past. It was those events that led to the city being corrupted by Chaos, etc. The army of Asavar Kul then moved on, was defeated at Kislev by Magnus the Pious, and the force of Chaos driven north of Praag again. The city was so corrupted that it had to be razed to the ground; the Kislevites rebuilt the city atop it, but still experience problems from time to time, and so occasionally portions of the city are destroyed and rebuilt.
During the Storm of Chaos, Praag was besieged by the forces of Archaon, but Kislevite and dwarven forces held the city and it never fell. Sometime after Archaon was driven out of Sokh, near Middenheim, the Chaos forces at Praag began to break up and the siege was lifted.