Lublin’s royal castle dates to the 12th century, though it's been rebuilt many times since; the oldest surviving part is the impressive Romanesque round tower that dominates the courtyard. It was here in 1569 that Poland's union with Lithuania was signed. The castle is home to Lublin Museum and the 14th-century Gothic Chapel of the Holy Trinity, which contains Poland's finest examples of medieval frescoes; admission gives access to both and also to the tower.

　　The 14th-century chapel is considered a masterpiece of the Middle Ages, with Russian Byzantine–inspired frescoes painted in 1418. They were later plastered over, but were rediscovered in 1897 and painstakingly restored over a 100-year period.

　　The museum's permanent collection features mainly art, folk art and weaponry. During WWII the occupying German army used the castle as a prison, holding as many as 40,000 inmates. The darkest day of the war here came in July 1944, just ahead of the prison’s liberation by the Soviet Red Army, when the Germans executed 300 prisoners on the spot.