Holland’s 19th century Fort Asperen armoury in the town of Acquoy near Utrecht might have held explosives in the past, but its delicate timber structure stands at odds with the impressive defence line it is part of – all of which is now a designated UNESCO World Heritage site. Permission for the armoury’s conversion into an art gallery stipulated that the original structure must not be touched, so architect Bureau SLA literally built another building within it, completely clad in white LG HI-MACS to create a clear material counterpoint to the old building. Not only that, but the firm excavated 3m below ground level to create a gallery space that is far larger than first meets the eye. The overall effect is of a dramatic insertion into the armoury that is as alien as it is Tardis-like.