The Castellania was built purposely as the law courts of the Order, having underlying shops. It is built on two levels and the chambers are arranged on three sides of a courtyard which is located at the rear of the building. Apart from offices and the court hall, there are also a chapel and cells for prisoners, the latter exhibiting numerous historic graffiti.

Grand Master Emmanuel Pinto demolished the old building of the Castellania, which was built by La Cassiere, and remodelled it in 1757 to the design of architect Francesco Zerafa. Ġiuseppe Bonniċi took over upon Zerafa's death in 1758, and the building was completed in 1760. The law courts were later relocated to the Auberge d'Auvergne. Part of the Castellania was then converted into a secondary school for girls.

The façade's concave central bay contains marble elements and allegorical statues of two females representing Justice and Truth which are the symbols of the Law Courts.

The missing bust and coat-of-arms may have been removed during the French occupation or during the governorship of Sir Thomas Maitland.

In the courtyard of the Castellania is a fountain consisting of a large semicircular stone basin with a square pedestal supporting a stone eagle in a sitting position.

At the corner of Merchants' and St John's Streets is a pillory (unique in Malta) set in the splayed corner pilaster of the Castellania. It consists of a stone column with the lower four courses of upper coralline limestone like in the rest of the façade of the building. Delinquents tried by the tribunal and condemned to endure the strappado were taken through the Castellania's side exit (in St John's Street) and escorted to the pillory. The condemned sat on the stone stool of the pillory and tied to the metal bar which is still in place and left exposed to public ridicule.

Mepa scheduled the Castellania including the fountain and pillory as Grade 1 national monuments as per Government Notice No. 276/08 in the Government Gazette dated March 28, 2008.