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. The underground basements are currently closed, however, the museum of the September campaign is open daily 10:00-17:00 (closed Mon), and finally, the viewing tower, showing the entire area and beyond, open daily 09:00-17:00 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.