Situated in the town of Nowy Dwór Mazowiecki, 30km north-west of Warsaw, is the 19th century Modlin Fortress (Twierdza Modlin), with a collection of barracks, which in total amount to 2,250m - the longest building in Europe. It is here at the meeting point of two rivers, the Narew and Vistula, Napoleon in 1806 ordered the creation of a defensive point (he is rumoured to have designed one of the first buildings). The complex was later expanded by Tsarist Russian troops and later again by Polish troops, and indeed, was also used by the Germans. During the Russian tenure, from 1880 onwards, the main fort at Modlin became part of a larger defensive project, consisting of two defensive rings, one smaller inner circle consisting of 8 fortlets, 7-10km from the main fort, and the outer ring made up of 14 fortlets, 8-12km away. The entire defensive complex eventually became redundant as military technology and strategies evolved in the 20th century.
The fortress saw major action in the Battle of Modlin, during the 1939 Defensive War after Nazi Germany invaded Poland. 24,000 troops held out from 13-29 September and were one of the last to capitulate. Post-war, the fortress was used by the Polish military, but now the fort has been sold to developers who plan to create a 'town within a town' with a shopping centre, among other things, planned to be here in the future.
Despite this, parts of the fortress are still open to tourists and you can visit individually or join a group. Eitherway, for history buffs and Urbexers, this is a great place to visit. Although entry onto the terrain is free, if you wish to explore certain areas, you will need to pay, depending on what you wish to see. You can visit the underground basements Sat/Sun 11:00-16:00 (5zł), the museum of the September campaign, open daily 10:00-17:00 (closed Mon), and finally, the viewing tower, showing the entire area and beyond, open daily 10:00-18:00 costs 10zł. The easiest way to get there is to take the Koleje Mazowieckie 'RL' train, which leave once an hour from 08:15, from Warsaw Central Train Station to Modlin, from where the fort is just a 10 minute walk to the west along the river. Please note: opening times may vary, check their website for details.