A crypt belonging to a secret order of knights hundreds of years ago, a brand new eco-energy centre and a fake medieval castle are among the buildings open to the public for one weekend only.

Open House, which takes place on September 21 and 22, is a yearly event that gives people the chance take a peek into some of the most prestigious, important or just plain interesting buildings across London.

And in Islington and Hackney, nearly 90 of the most intriguing constructions in the boroughs fling open their doors to the curious crowds. Here are some of the highlights.

n The Museum of the Order of St John, in St John’s Gate, Finsbury, dates back to 1504, with the crypt of the priory church dating all the way back to the 1140s. It was the former home of the Knights of St John – a group of monks who gave medical aid to pilgrims in Jerusalem and took a military role after crusaders took the city. Today, their name is used by the St John ambulance service.

Benefits

n The Bunhill Heat and Power Energy Centre, in Central Street, Finsbury, is a ground-breaking energy scheme designed to take power away from the big five energy companies, as well as reduce fuel poverty and yield financial and environmental benefits to the community.

n The Castle Climbing Centre, in Green Lanes, Finsbury Park, is a water pumping station built in the form of a mock medieval castle. It has now been converted and is popular as an indoor climbing centre.

n W. Plumb Family Butchers, Hornsey Road, Upper Holloway, is a Grade II listed, ornate former Edwardian butcher’s shop with art nouveau wall tiling, geometric tiled floor, scrolled meat rails and mahogany cashier’s booth with etched and brilliant cut glass.

n Village Underground, in Holywell Lane, Shoreditch, features a recycled Tube train – a well known landmark – turned into artists’ studios on reclaimed land on top of an abandoned railway viaduct.

n Tramshed, in Rivington Street, Shoreditch, dates from 1905 when it was built as an electricity generating facility for the tramway system. Andrew Waugh will explain how he managed to transform this Grade II listed building into one of London’s most talked about restaurants, complete with a Damien Hirst installation.

n Sutton House, in Homerton High Street, Homerton, is a National Trust building and a rare example of a Tudor red-brick house in the East End.

n The Priory Green Estate, in Collier Street, King’s Cross and the Spa Green Estate in Rosebery Avenue, Finsbury, both have notable features; Priory Green’s pioneering use of concrete and sculptural stairways, while Spa Green has been described as the most important post-war modernist housing development.

n The Marx Memorial Library, in Clerkenwell Green, Clerkenwell, was built as a Welsh Charity School but has been a library since 1933. Lenin worked here from 1902 to 1903 and his office is preserved.

n Ironmonger Row Baths, in Norman Street Finsbury, is a Grade II listed building that underwent a multi-million pound revamp completed last year, with original features retained along with Turkish baths, laundry and swimming pool.

n Hoxton Hall, in Hoxton Street, Hoxton, is a surviving Grade II* listed Victorian saloon-style music hall dating from 1863.

n Clissold House, in Clissold Park, Stoke Newington, is a recently restored late 18th century brick villa set in parkland and a rare survivor of its type.

n Circus Space, in Coronet Street, Hoxton, is the UK’s national centre for circus arts housed in the former Shoreditch Electricity Generating Station, opened 1896.

n Fifteen Buckingham Road, in De Beauvoir, is a refurbished Victorian semi detached house with contemporary rear extension and a semi underground timber clad cylindrical drum for the lounge, home office and library.

n For a full list of all buildings taking part in both boroughs, visit www.islingtongazette.co.uk and www.hackneygazette.co.uk