The Old Royal High School is an alternative history of Scotland written in neoclassical stone. A monument to aborted plans and dreams postponed, it looms from the south slope of Edinburgh’s Calton Hill. For half a century, it has existed in some sort of limbo, a state of loss and latency, after 140 years as a school it was converted as a dreamed-of home for a devolved Scottish Assembly which didn’t happen when the referendum failed in 1979. It was renamed New Parliament House, a title whose potential was finally dispensed with when devolution came in 1998 and plans were made for a new building at Holyrood.

Years followed in which it was like a sleeping giant, shaken-awake now and then with some fresh new plan – it would be a national photography centre, a hotel – or temporary function for Edinburgh Council, which owns it.

No room speaks more of its place in Scotland’s history than the debating chamber, built in the great hall in the 1970s. The school was a space of potential that would be considered too small, too inaccessible, too inappropriate by the time devolution finally arrived.

It’s exactly this kind of lost, liminal space, halfway towards dereliction or desertion that the Hidden Door festival, which started last week and runs till June 18, tends to make its home. In years past it has colonised the former Leith Theatre, the Market Street vaults, the old lighting depot on King’s Stables Road, and now it moves into the Old Royal High. For the most part it leaves behind a trail of reconnection and renewal. That renewal was already on the cards at the Old Royal High, long before the festival arrived this year. Before the Pianodrome was installed and the music and art line-up announced, plans to turn the school into a National Centre for Music were given unanimous approval

Hidden Door breathes life back into spaces – and this year it is doing that with music that includes Saint Etienne, Yellow Days and Warmduscher, as well as visual art, dance, theatre and spoken word. The festival’s creative director, David Martin, said: “It feels really exciting, in what is still a very challenging period for the arts, to invite Scotland’s newest, most daring artists, dancers, theatre-makers, poets and musicians to take over such a prestigious building that has been dark for so long and flood it with creative energy.”

A marvellous prelude to what is to come – the long-term reanimation of this sleeping giant with music.