The Royal Castle (Zamek Krolewski) was built in the 13th century, initially as a duke's residence and from 1526 as the king's residence. After the capital was moved from Krakow to Warsaw the Royal Castle also became the center of the executive and legislative authorities.
In 1791, the May 3rd Constitution, Europe's first modern codified national constitution as well as the second oldest national constitution in the world, was drafted here.

It was the royal palace and official residence of the Polish monarchs until 1795 when the Poland was partitioned between Russia, Prussia and Habsburg Austria.

It is also here, that during the November Uprising, in 1831 Polish Parliament removed Tsar of Russia Nicholas I from the Polish throne. Russians rebuilt the castle in the years afterwards, demolishing Sejm halls, trying to uproot all traces of Polish democracy.

Between 1926 and World War II the palace was the seat of the Polish president.
Partially destroyed by German bombers during the Invasion of Poland in September 1939, it was mined and completely damagedd by the Germans after the Warsaw Uprising (1944).

Reconstructions from rubble started in 1971. In July 1974 the clock on the tower began working again, on the exact same time at which it was stopped by the Luftwaffe bombardment. The renovation was finished in 1984.
Nowadays the Royal Castle is a monument of Polish Culture and Heritage and it is used as a branch of the National Museum.

More info here:
http://en.wikipedia.org/wiki/Royal_Castle,_Warsaw