Court of arches.

A distinguished seventeenth-century citadel has been given new uses, enhancing its original qualities, and allowing the new work to be seen for what it is.

The Petit Arsenal is in the middle of Vauban's seventeenth-century fortress which commands the city of Besancon and the valley of the Doubs, down which armies from the east would have had to march through the Franche-Comte to seize Burgundy and the heart of France. For some decades now, the huge citadel has been redundant in a military sense and it has been converted to a variety of educational and cultural uses, notably the museum of natural history.

The lower level of the Petit Arsenal was converted to house the museum's aquarium in the 1970s. This was seen to be out-of-date by the early '90s and Bernard Quirot with Olivier Vichard were appointed to make a new exhibition arrangement and simultaneously bring out the qualities of the old buildings. They seized the opportunity of turning the central court into a garden for displaying the local fauna and flora. With landscape architect Philippe Convercy, they decided to divide the court with four parallel hardwood decks which are joined by steel bridges in staggered pattern, so discreet rectangular beds are formed, in each of which a different culture can be created. One of the rectangles is a pool with aquatic plants; another is a raised tank for fish, made of concrete with one glass wall: detailing is simple, clear and elegant, plainly influenced by the work of Scarpa.

This approach is carried over into the treatment of Vauban's Petit Arsenal. The four arches of his arcade have been enclosed by screens of glass and hardwood (iroko like the slats in the external decks, but oiled to a deep brownish yellow). This sounds like a dumb move, but it has been carried out with grace. The split cruciform design of the screens is generated in response to the springing and keystones of Vauban's arches. The screens are set well back so that the old stone is allowed to speak for itself, and the new work is seen for what it is, a set of well crafted additions subservient to (and removable from) the frames in which they are set. The northernmost screen folds open as a pair of entry doors, but the others are fixed as windows and display cases (the architects did not have anything to do with the rather dated graphics).

Internally, partitions between the old vaults have been swept away, allowing the whole fine space to be seen as one. Three big tanks dominate the back aisle, while the front one is largely left open, so the space can be appreciated as one volume. Here, as elsewhere, the old work is respected and enhanced, and the new is complementary but not subservient to it.