Visby is an old hanseatic town on the island of Gotland in Sweden. The town has a large number of old church ruins, one of which goes by the name of St. Lars. The church is believed to be a 13-century orthodox church, and abandoned in the 16 century, all that is left today are the stone walls and parts of the inner ceiling vaults. Through collaboration with the local museum, St.Lars has now been measured and 3D-modelled by the author, Sebastian Holm from Efterklang, who is also a part-time musician. The model has been fitted with what is assumed to be an historically accurate ceiling structure and materials as well as windows, doors, various furnishings and a make-up stage. With acoustical modelling and auralisations made in Odeon, various source and receiver positions has been tested for acoustical qualities, and the impulse responses are now used for musical production for the medieval band known as Patrask. The mixing process uses the impulse response from left and right side of the stage to produce a stereo reverb, and the results are compared to auralisations of the music made with Odeon. The overall process is discussed, with links to the music itself.