Girolamo Cassar was a Maltese architect and military engineer. He was the resident engineer of the Order of St. John, and was admitted into the Order in 1567. He was involved in the construction of Valletta, initially as an assistant to Francesco Laparelli, before taking over the project himself. He designed many public, religious and private buildings in the new capital city, including Saint John's Co-Cathedral, the Grandmaster's Palace and the auberges. He was the father of Vittorio Cassar, another architect and engineer.

The Order of Knights of the Hospital of Saint John of Jerusalem, commonly known as the Knights Hospitaller, is a medieval and early modern Catholic military order. It was founded in the crusader Kingdom of Jerusalem in the 12th century and had headquarters there until 1291, thereafter being based in Kolossi Castle in Cyprus (1302–1310), the island of Rhodes (1310–1522), Malta (1530–1798), and Saint Petersburg (1799–1801).