St. John's Co-Cathedral

Where it sits. The Co-Cathedral stands on St John's Square, a small stepped piazza off Republic Street in the middle of Valletta, one block from the Grand Master's Palace. Anyone walking past it without a guidebook would take it for a barracks or an arsenal. The facade is severe grey globigerina limestone, flanked by two squat bell towers, with a shallow balcony over the door and almost no ornament. This was deliberate. It was built as the conventual church of a military order in a city that had just survived a siege, and it was designed to look like it could survive another one.

Who built it and when. The Order of St John commissioned it in 1572, six years after Valletta's foundation, and Girolamo Cassar, the Maltese architect who had taken over the city's works from Francesco Laparelli, completed the shell by 1577. Cassar gave it a single barrel-vaulted nave about 53 metres long with side chapels opening off both flanks, one for each of the eight langues, the national divisions into which the knights were organised: Provence, Auvergne, France, Italy, Aragon, Germany, Castile-Leon-Portugal, and England, later replaced by the Anglo-Bavarian langue. Each langue decorated and maintained its own chapel, and they competed. The result is a single interior that reads as eight parallel arguments about status.

The interior. Nothing outside prepares you for it. In the 1660s, under Grand Master Raphael Cotoner, the Calabrian painter Mattia Preti was brought in to redesign the whole interior. He had the plain stone walls carved in deep relief and gilded, so the surfaces themselves are cut limestone rather than applied stucco, and then he painted the entire barrel vault himself in oil directly onto the stone between 1661 and 1666, with eighteen scenes from the life of St John the Baptist. Painting in oil on porous limestone required sealing the surface first; the fact that it has survived three and a half centuries is a technical achievement as much as an artistic one. The floor is the other shock: roughly four hundred tomb slabs of knights, each an inlaid panel of coloured marble intarsia carrying heraldry, trophies, skeletons, hourglasses and Latin epitaphs in a sustained exercise in memento mori. Visitors are given a strict instruction about footwear, because stiletto heels punch through marble.

Caravaggio. The Oratory, entered from the nave, holds The Beheading of Saint John the Baptist, painted in 1608 and roughly 3.7 by 5.2 metres, the largest canvas Caravaggio ever produced and the only painting he is known to have signed. The signature is worked into the blood running from the Baptist's throat. Caravaggio had arrived in Malta in 1607 as a fugitive from a killing in Rome, was received into the Order as a Knight of Obedience in July 1608, was imprisoned at Fort St Angelo after a violent brawl within weeks, escaped the island, and was expelled from the Order in absentia that December in terms describing him as a rotten and fetid limb. His Saint Jerome Writing hangs in the same room. Both were painted for this building and have never belonged anywhere else, which is rare for works of that stature.

The other treasures. The museum wing displays a series of large Flemish tapestries woven in Brussels after designs by Rubens and Poussin, presented by Grand Master Ramon Perellos at the turn of the eighteenth century and still hung in the nave for the feast of St John in June. The Chapel of the Blessed Sacrament keeps silver gates that, according to the story told on every tour, were painted black so that Napoleon's troops would overlook them when they stripped the church of its precious metal in 1798. Grand Masters are buried in the crypt, including Jean Parisot de Valette, founder of the city, and Philippe Villiers de L'Isle-Adam, who brought the Order to Malta. Pope Pius VII raised the church to co-cathedral status in 1816, sharing the seat with the cathedral at Mdina, which is why the odd title.

Getting there and timing. The entrance is on Republic Street and the exit puts you out on Merchants Street. Tickets include an audio guide, and there is a separate marked route so that people move through the Oratory in one direction. Go on a weekday morning as soon as it opens; by mid-morning cruise groups fill the nave and the Oratory becomes a slow queue. It closes to sightseeing on Sundays and public holidays, when it functions as a working church. Photography is permitted in the nave but not of the Caravaggio.