A sandy land difficult to farm, this part of Poland might have remained obscure except for a twist of fate: Poland teamed up with its eastern neighbor Lithuania in the 16th century and needed a political meeting point closer to the Lithuanian capital in Vilnius. It found it in Warsaw, and from that time on this region became the center of the country.
Outside of the nation's capital, you can explore one of Tylman van Gameren works for the powerful Lithuanian family, the Radziwll, in Nieborow. For another glimpse of castles, try the Romantic version in Opinogora which fittingly houses the Museum of Romanticism. If something more current is to your taste, visit the city of Lodz. Depicted in Wajda's film The Promised Land, Lodz gives you a taste of life in industrialized Poland but do not expect any hidden pockets of aesthetic beauty that you can find in other Polish cities. For another sense of commerce, visit the market town of Pultusk, where trading has gone on for centuries.
Mazowsze also offers something for the religious: it is here that the line of the Hasidic Alters began in Gora Kalwaria; commencing with Rebbe Meir Alter, his grandson managed to make it to Israel and continue there. On the sadder end of Jewish history in Poland, you can pay respects at the concentration camp Treblinka. For a different spiritual flavour, enjoy the folk culture in full form during the festival of Corpus Christi in Lowicz where the Archbishops of Gniezno traditionally resided. Or explore the 12th century collegiate church built in the Romanesque style in Tum.