Castle keep.

This renovation of a medieval castle in Trevi sensitively inserts contemporary elements within a historic shell.

Carlo Scarpa's influence on restoration of old buildings has been almost entirely benign. The mixture of tender and understanding repair of old work with clear modern interventions in twentieth-century materials, which Scarpa demonstrated so imaginatively at the Castelvecchio in Verona, has been echoed all over Europe.

One of the most convincing recent examples of the approach is the Caetani castle in Trevi. It was one of the medieval fortresses which defended the Via Latina in the lower Lazio, the region immediately north of Rome. Dominating the still largely untouched dense pattern of vernacular streets and houses of the old town, the castle (built on the site of a Roman fortress) had fallen into great disrepair, with only the partly decayed stone keep and curtain wall remaining. Over the centuries, all the timber parts (including the upper floors) had rotted or been looted, leaving one stone shell within another.

Gianfranco Cautilli, Mario Morganti and Renato Morganti were asked by the municipality of Trevi to convert the ruin for use as the local archaeological museum and visitor centre for the Simbruini mountain national park. The architects' fundamental strategy was to repair the broken stone shells back to their original skylines using local porous limestone. The stones will probably weather until they are the same colour as the medieval ones, but there is no attempt to make the new work look like old; pointing is in proper mortar with raked joints, yet the stones do not emulate the original shape or bond.

And there has been no attempt to restore missing wooden floors or frame openings. Levels are very complicated, particularly clown at the Roman top of the mound. They are related by new stairs, external ones with stone treads, internal flights with timber. A bridge connects to the keep from the perimeter accommodation. All these new pieces have steel structures: rolled steel members are under the bridge and stairs; flights are suspended on steel rod hangers from I-beams which span between the stone walls. Balustrades are of glass within steel frames, black like the beams and stringers. Detailing throughout is precise, economical and elegant. Roofs over the keep, and over the gallery which links the upper levels of the accommodation within the curtain wall, are industrial decking over timber members which bear onto steels between stone.

Contrast between old and new is most dramatic in the keep. What had been a stone chute of space open to the sky has been lidded, and is now inhabited by the hovering presence of the steel and wood stair which is adjusted to bring you out to landings that offer views through original openings in the massive walls. Similar prospects over town and country must have been familiar to the Castilians and their chatelaines. Throughout, you are made aware of the past while standing on the present; you are invited to make your own commentary on the original work and the often moving changes which have occurred over the life of the building. Such interpretations would have been completely impossible if there had been an attempt to restore the castle to its medieval state: in any case, there can be no definitive version of a building that has continuously changed over a millennium. Now it has altered again, and will doubtless do so in future. But the new work does not prejudice what will be passed on while in our time it adds to what we have inherited.