The Grand Master's Palace (Castello and Palazzo) is beyond doubt the most emblematic work of Gothic architecture in Greece, and a symbol of the medieval town of Rhodes. A building of imposing dimensions, it dominates the highest point on the north-western side of the castle, where the Street of the Knights ends. It was built in the ancient citadel, on the site of the ancient temple dedicated to the sun god Helios. The palace was first founded at the same time as the Byzantine walls, which became necessary because of repeated Arab-Persian raids from the mid-7^th century on.

The Knights of St. John erected the Castello in the 14^th century to house the city’s military and administrative centre. Under Ottoman rule, from 1522 onwards, the palace functioned as a prison and was left to fall into decay. It was almost entirely demolished by an explosion in an adjacent gunpowder magazine in 1865, and owes its present form to extensive restoration projects carried out by the Italians in the 1930's. It is a tower-shaped rectangular building (approx. 80 x 75m) with a large paved courtyard (approx. 50 x 40m) surrounded by galleries. One of the palace’s most distinctive features is the main gate to the south, flanked by two tall, strong round towers. The ground floor had auxiliary rooms, while the four wings of the first floor had over eighty or more rooms, as well as halls decorated with mosaics and frescoes . The most impressive rooms are the central council room, the refectory and the chapel dedicated to St. Catherine.

Since 1993 the Medieval Museum of Rhodes has been housed in seven halls in the south-west wing of the Palace of the Grand Masters. The permanent exhibition, entitled "Rhodes from Early Christian Times to the Ottoman Conquest (1522)", is divided into seven rooms on the following subjects: (a) Introduction, from Ancient to Christian Ideology; (b) Economy; (c) Social Life; (d) Defence and Administration; (e) Intellectual Life; (f and g) Worship and Art.