When we sit down to talk with Heidi and Jackie, the owners of Locked In Edinburgh, they have just returned from a trip to Hamburg. They'd gone to play the escape rooms at Skurrilum, ranked world number-one, and the sheer thrill of it still has them aglow with happy enthusiasm. 'It's just blown our minds,' says Jackie. They won't give away any details, but recalling its twists and turns has them bursting into peals of laughter.

Such open passion speaks to their own origins as escape room designers which, they acknowledge, don't conform to the stereotype: while many come from a background of theatre or design, the two of them began simply as fans of the game. 'We came out of the first room we played, our hearts pounding: that was brilliant, so clever!' says Heidi. 'And then we went to play another one, which was also good, but it made us think, "We could build that better."'

As the managing director of Daybreak, which has offices operating out of Summerhall, Heidi knew that the historic multi-arts venue had a space available, and pitched the idea of turning it into a pop-up escape game. Summerhall bequeathed it to them for twelve weeks in the spring of 2015: Heidi and Jackie spent eight of them designing their first room, and ran it for four, before relinquishing the space back to Summerhall for the Fringe. 'Then after the festival,' says Heidi, 'they came back to us and said, "Would you like to come in permanently?"'

The Cutting Room

Four years on, their one room has expanded into three immersive games: The Secret Lab, The Cutting Room, and the Distillery, each with their own distinct character. Last year, the Secret Lab was voted as the Best Room in the UK, and 13th in the world. This accolade is particularly significant, given that Locked In operates at a fraction of the cost of most mainstream escape rooms. When asked about the strengths which separated them from the rest of the pack, Heidi and Jackie are quick to mention the strong symbiotic relationship between Locked In and Summerhall: for instance, the vast majority of their rooms' set dressing are salvaged from the site. As a former veterinary school, there is no shortage of interesting pieces to discover, if one has the know-how and the imagination.

'I'm usually holding onto Heidi's leg as she's climbing into a skip, when someone in the building has thrown out something that we can use,' says Jackie with a laugh.

'Someone else will see an old piece of metal and we'll think, that's a blood transporting unit device!' says Heidi, 'or, that's a decontamination unit!'

'We had to fight for that one,' Jackie recalls. 'Someone else was wanting to make it into a barbecue, and we were like, "no it's ours!" We just managed to get it up the lift before they took it.'

Locked In Edinburgh

The unique character of Summerhall itself has also lent them a competitive edge. 'We build around the existing atmosphere that no other game can offer, because they're in man-made offices,' says Jackie. 'If we keep true to the building, it helps people with the immersion. When we took over the rooms, we thought about painting everything and making it feel new, but we realised that we had this special, old atmosphere and we don't want to change that.'

Everything in the rooms have been built within Summerhall's walls, either by Jackie and Heidi themselves, or from drawing upon the rich pool of talent and expertise housed within the venue, such as the tech wizards over at the Hacklab. This organic, people-built approach has allowed them the space to be more innovative, more ingenious than their competitors. 'We want to make do with things that we've built, and we want to build them better,' says Heidi. 'So that hopefully we can touch all the elements that make an escape room great. It's very much about: how did we feel when that happens? What was the emotion? How can we get that emotion across?'

Creating from scratch has also allowed them to invest an incredible amount of detail into their rooms. For instance, there was a debate over getting the smell right in the Cutting Room: an operating room should, in reality, be sterile and thus smell like nothing, but they ended up settling for the smell of Dettol since, they figured, people are quicker to associate that scent with the doctor's office. Such attention to detail speaks to their deep love of the game, and an understanding of what people love most about escape experiences: the immersive narrative, the sense of reward and fulfilment – the escape, essentially, from the everyday frustrations of life, as everything is reduced to the puzzle before you.

The Secret Lab

'People have asked us, "oh, do you go play other rooms to copy other ones." Not at all,' says Jackie. 'We go back to remember that feeling of being a player, of being really stuck, really frustrated, and then – if the room is designed well – that flow which takes you in the right direction, and coming out on a high of having received reward after reward.'

'If they're feeling unfulfilled when they leave, then you haven't done it right,' says Heidi. 'From the minute you meet them in reception and give them the briefing, they should be anticipating going into that room, they should have that "wow" moment, the ups and down, a proper narrative journey, and come out feeling like, oh my god. Like you've achieved something.'

Despite their accolades, Heidi and Jackie are certainly not ones to stay still, and are already looking ahead with ideas for longer, stronger narratives and more immersive experiences around different locations in Summerhall. And now, four years into their journey from fans to accomplished designers, are there any words of wisdom would they give for those looking to escape?

They look at one another. 'Communicate with each other,' says Jackie, 'say what you see.'

'Communication is key,' Heidi agrees, 'but also, if you can't do something, let someone else do it. Walk away.'

'Make a to-do pile and a done pile, and bring everything to a common place.'

'If you have a lock with a key, once you've opened it, leave it in the lock so you don't find that key again or try to use it.'

'Mostly, just enjoy it,' says Heidi.

'Have an open mind,' says Jackie. 'You'll surprise yourself.'