Treasure chest: Castellon's museum is conceived as a hermetic, almost impervious aluminium chest that guards the city's fine collections. Inside, it allows its treasures to be eloquent.

Madrid-based Luis Mansilla and his partner Emillo Tunon began their careers in the office of Rafael Moneo and their work is strongly inflected by similar formal concerns. Neutral, toplit containers, solid, alcazar-like walls and the subtle play of light are intelligently choreographed to create a sense of depth and solidity. Critic David Cohn notes that 'They profess a formal restraint which recalls the late Spanish master Alejandro de la Sota. As they themselves describe it: "Architecture isn't exactly silent. It is more like a conversation in lowered voices. Ideas are present, but the true effort lies in making them invisible".'

One of their largest and most recent projects is the new Museum of Fine Arts in Castellon, Surrounded by undistinguished mid-rise apartment blocks dating from the 1980s, the museum lies, unusually, in a residential area on the edge of town. Its collection is diverse, ranging from Roman archaeological specimens to paintings by local artists such as Francisco Ribalta and Jose Ribera. Incorporated into the new complex are the surviving chapel and cypress-filled cloister from a Catholic school (the Serra Espada) that originally occupied the site.

These existing historic fragments have been joined by two legibly contemporary new additions. To the east of the cloister, a five-storey cubic volume, sphinx-like and inscrutable, houses the new exhibition spaces and forms the public focus of the museum. To the west, a long low bar contains restoration studios, workspaces and storerooms, marking out a more private, specialist domain. In between, the restored courtyard building mediates between the two realms, with a library, auditorium, offices, technical spaces and a long gallery for temporary exhibitions arranged around the cloister. The three volumes are linked by a plith, creating a gradient of intimacy between the public areas to the east and the workshops on the western boundary.

Conceived as a hermetic chest guarding the city's treasures, the new exhibition block is a sealed, impassive cube topped by rows of tall rooflights. The geometry of the rooflights gives the building a crenellated profile so it looms over its surroundings like a brooding castle battlement. The same serrated silhouette rounds off the long workshop volume on the opposite side of the cloister. This slightly daunting mood is lifted by playful superscale letters spelling out MUSEU supporting a canopy that defines one edge of the entrance parvis. (A giant photograph of the individual letters being transported on lorries barrelling along the motorway from Madrid forms one of the museum's more surreal exhibits.)

The idea of the building as a chest or case is further elaborated by its enveloping metal carapace made from panels of recycled aluminium. Two kinds of panels are used, one opaque with vertical ridges, and the other resembling a veil, with horizontal louvres that allow light to filter into the gallery spaces. The narrow borders of each panel are stamped Museu de Bellas Artes, a mark that identifies the building for which they were made, in the manner of the Roman bricks in the museum's collection. The horizontal and vertical striations of the aluminium panels create a strong geometrical rhythm and texture, softening and breaking up the immense mass of the exterior. Seamed zinc sheeting is used to clad the opaque parts of the roof.

Spread over five floors, the museum's collection is divided into four themed sections (fine arts, craftwork, ceramics and archaeology). Rooms are linked by a series of double-height voids that cascade down diagonally through the interior. Drawn upwards from floor to floor, visitors are simultaneously confronted with three different scales and aspects of the building: individual rooms, the double-height voids and a dramatic diagonal prospect of the entire volume. A 7.3m x 6.6m grid, common to both plans and sections, ensures formal coherence. The spatial sequence is crowned by the parallel skylight bars which diffuse a soft luminance into the gallery spaces. In places, milky glass external walls give diffused views of the louvred metal skin beyond. A spirit of elegant materiality prevails (another characteristic of Moneo), with benches and display cases fashioned from modular assemblies of iroko strips and thin sheets of glass. Calm and ascetic, the interior acts as a discreet backdrop for treasures on show. Like Portia's lead casket in The Merchant of Venice, Mansilla + Tunon's sober metal container conceals a radiant richness at its heart.