At Oxford Castle, a body lay in a crumpled heap. One passing Christmas shopper did look quite concerned, and paused briefly before hurrying on. Nobody else took any notice at all. Admittedly the body was dressed as a 12th-century knight, so perhaps they all thought it was too late to call the police.

George the knight (Jeremy Allen) was supposed to be guarding Empress Matilda while she was under siege at Oxford Castle — a moment in local history that’s the subject of UnderConstruction Theatre’s none-too-serious new play Mummers, Michaelmas and the Empress Matilda. As with the company’s Mapping Wonderland last summer, you are taken on an hour-long promenade, and various quirky characters leap out of the shadows. First it’s the aforementioned George (still alive at this point) spouting decidedly rough poetry under the windows of the Malmaison Hotel — perhaps to the consternation of guests, who might think they’d fetched up in Stratford by mistake for sub-standard Shakespeare. Worry not, guests, George is soon silenced. Talking of Stratford, as usual with UnderConstruction shows, you next come across a dodgy shop. This one sells “slime from the Avon”.

The proceedings are conducted by three irrepressible jesters: Marcus Davis-Orrom, Devane Boyd, and Guy Killner. They are extremely good at interacting with the audience, and ad-libbing as required — at the performance I attended, they had to rise above a noisy fight, not between Matilda’s men and King Stephen’s advancing army, but between two small dogs.

Matilda herself (Bryony Neilson, above) is a chilly character. She looks disdainfully at her Christmas presents of a white cloak and matching boots, even though they act as life-saving camouflage for her escape through the snow. Perhaps it was her rather quiet voice, but I didn’t even hear a “Thank you”. Tut, tut. Nonetheless this show certainly provides an entertaining short break from the Christmas slog.

There are further performances from Thursday to Sunday until December 18. Advance tickets: gigantic.com or from the O3 Gallery at Oxford Castle.