Cloistered creativity.

On a site in Spain where medieval penitents once made more spiritual journeys, Alvaro Siza has created an architectural promenade of elegant functionalism in this unashamedly Modern gallery for contemporary art.

Since the early Middle Ages, the Shrine of Saint James at Compostela has been the focus of pilgrimage from across western Europe. The landscapes of France and Spain are lined with hospices and high crosses marking the penitent's route to Santiago. Now, in this new Europe of the Regions, the Galician capital is implementing an ambitious plan of growth, preserving the mini-fortresses of its ecclesiastical core and (under the tutelage of its Mayor, architect Xerardo Estevez Fernandez, and J.P. Kleihues) substantially extending its periphery with contemporary usages. On the edge of the old town, Alvaro Siza has recently constructed the Galician Centre of Contemporary Art with in the redundant monastic property of San Domingos de Bonaval. The great achievement of Siza's project is to be both appropriately contemporary and unmistakably of its place. Siza has a stellar reputation as critical regionalist with a built oeuvre that combines abstract and international tropes with indigenous realities. At Compostela (field of stars), he heads a prestigious list of invitees, which includes Grassi, Hejduk and Viaplana Pinon, who are inserting twentieth-century architecture into a massive archaic fabric. His total project is the walled precinct of the monastery, the principal edifice of which is the independently realised Museum of the Galician People. At the time of writing, minimal interventions to the enclosed cemetery and gardens (portals, paved thresholds, geometric planting) are almost complete with a site on the farthest hill awaiting a second Siza design. His Centre of Contemporary Art is deliberately low, next to town, fortifying the boundary of San Domingos and a damaged side-street named after Ramon del Valle-Inclan, Galicia's most famous writer. The gallery nudges south to present its sculptural portal in a small plaza between town and monastery, heightening one's sense of arrival from urban density to semi-rural compound.

Siza starts with horizontal masses shifting beneath the signature elements (tower, gable, gateways) of the monastery, the new bulk subsuming itself geologically beneath the silhouette of the old. Two controlling axes are established alongside and rectifying rua Valle-Inclan and, at a 21 degree rotation, parallel to the linear walled cemetery piercing deep into the total site. This principal geometric system originates physically and metaphorically from the small stepped forecourt without being explicitly stated. The new architecture holds itself back in respect to the old and enjoys a kind of blunt sectional resolution. Excavating his stereometric form at the southern tip of Valle-Inclan, Siza creates a grand orthogonal canopy, open to town and monastery but sealed -- except for an iconic steel-supported strip -- to the plaza. Finally, the parti is set by rising up into the arrival porch so that one enters between an upper rooflit floor and a lower level eroded and swelling into a surprisingly expansive base.

This is an unashamedly Modern work. Growing from the broader monastic strategy and ameliorating an immediate context of urban route, place and edge, Siza's building is further engendered by a borrowed palette of granite and plaster. From within his portico, the building's flanks are almost unrelievedly of granite, bright but streaked and occasionally smeared. As the typical streets of Santiago are formed of contiguous stone slabs (kerbless, hollowed by wear, and in places diagonally set) so the gallery's outer envelope is wrapped, tautly but with exposed joints, in the same material (a lapidary Christo) with main expanses of glazing, as at the ramp from Valle-Inclan, recessed as subtractions from the primary form. At the triplicate entrance doors, such grey hardness switches to a luminous inner world of traditional white plaster and brilliant Greek marble. The exterior responds to the temperate climes of northwestern Spain -- sun and rain -- while the interior appropriates that light and directs it with minimalist artistry.

Siza's two blocks come together at the foyer but their meeting is screened or evaded by a segment of wall. As if to negate the weight above, energy is at both peripheries, to right and left, as panoramic windows towards the park and over the ramp onto the street advance into the volume spectacularly admitting east and west light. The former indentation overlooks a small skewed pool aligned with a chain of water features up through the terraced gardens, perpendicular to Valle-Inclan. The latter backs a serpentine desk of silver marble leading inwards where the intersection of Siza's axial systems is revealed in a triangular ceiling void with indirect zenithal illumination and rectangular upper openings. This is the inside flank of that block against the park, a block with an enfilade of permanent exhibition rooms above and temporary galleries below. A great vacant opening on to the pivotal triangular void punctures a parallel subsidiary spine of ramps and stairs connecting all levels. Into the crossover between these systems Siza locks his fluid movement pattern.

To the original spiritual ambulatory of medieval penitents, and to the shelving planes of his garden topography, Siza now contributes the architectural promenade of elegant functionalism. The reality of both linear blocks turning in towards each other as Ls -- the northernmost appendage houses auditorium with library above -- settles any headlong axiality and instigates a transverse meandering. One circulates up and down, back and forth, in a largely introverted world where one room leads to another unexpectedly and so comfortably that there is only the mildest sense of designer labyrinth or of curatorial diktat. The roof of the more bureaucratic wing along Valle-Inclan is an unexpected enclosed room; a sculpture court, open to the sky. But even this memorable space is too interconnected (geometrically and visually) to register as a compulsory terminus. Amid dynamic shards of wall and selected glimpses to the interior, an homogeneous granite slipway rises to views of the entire city. Repositioned thus above the entrance are recollections, in the parapet datum and the slightly surreal pyramidal rooflight, of Le Corbusier's Beistegui apartment and -- with the peripheral vitrines below -- of the Casa Malaparte on Capri.

Siza's Centre of Contemporary Art has a sense of containment and of release which is both physical and perceptual, both plastic and luminous. A visitor's route through the internalised galleries is marked by a limpid wash of light tempered by hovering extrusions of ceiling plane and then by moments of brilliance when natural light seeps into the geometric construct. From the foyer, light bounces off the ramped parquet floor of the auditorium and the underside of an interstitial stair to the office wing. Simultaneously, there is the non-presence above of still, almost frozen light in the recessive void of the triangular hall. In the walls of internal circulation, large rectangular cut-outs create the kind of mute enigma associated with artists such as James Turrell, framing subsidiary voids and the light or colour they might contain. Openings to the exterior, from the cafe to park and out on to the roof, are similarly of mullion-less glass and plain-white (chameleon) frames so that Siza's interior architecture may be said to be primarily constructed of light itself.

This is the gallery as viewed object, when approached from the town, and as viewing mechanism when experienced within. The visitor discovers the resolution of geometries, the cellars and the attic terrace, the hidden rooms such as the square chamber in the basement with its central mushroom column (an homage to Stirling?). Along the route are certain incidents fortifying the geometries and materiality- the glass-to-glass wall junction at the lecture theatre, the marble apex inside the office corridors, the boardroom antechamber tilted into the slot above the entry ramp. The overriding sensation is of the culling of unnecessary addition, of the negation of gravity, and of subtraction. Thus the timber and marble furniture in the cloakroom and cafe shifts and floats, without trimmings, as pure Suprematist form. And the skirting of the gallery walls is recessed so that they too float with service fixtures pressed back to a minimum.

Is the Santiago gallery a masterpiece? That last word is almost meaninglessly over-used, every city in the capitalist world proclaiming this or that designer pile as the ne plus ultra of achievement. This construction does, however, merit praise of the highest order. Resisting final judgement until the installation of artworks and its concomitant opening to the public, the Santiago project presents a complete range of architectural solutions from the urban and contextual to the juncture of materials and control of light. One might quibble about the necessity for the twin vertical slabs at entry or the dowel-like handles of the galleries' sliding doors but such tiny doubts dissolve in the evolving spatial experience that the architect has given us. In the medieval sense of masterpiece as an object prepared for admittance to masterdom, Siza has undoubtedly joined the master builders.