Around the time of Napoleon, what cities in modern Poland's borders could be considered significantly fortified to withhold month long sieges?
Zamosc, a private town and fortress built by the magnate family of the Zamoyski was one of the most important fortified city in Poland in Napoleonic times. It is known for the siege of Zamosc of 1813, when the Polish garrison for 8 months defended the fortress from the Russians.
Krakow was a fortified city which played a small role in the campaign of 1809 as the main goal of the polish offensive and the russian intervention.
Two others cities in modern Poland are famous to have resisted to Napoleon during the Prussian Campaign of 1806-1807, Kolberg (Kolobrzeg) and Dantzig (Gdansk). Marechal Lefebvre best known for his wife was the french commanding officer during the siege of 1806-1807. These two cities were the only points of resistance in Prussia after the battles of Iena and Auerstadt after which the Kingdom of Prussia collapsed and the whole frederician rotten structure crashed down and its armies surrendered everywhere in mass.