The rotunda of St Martin is the largest and oldest preserved rotunda in Prague. It was built in the second half of the 11th century. It has escaped demolition several times in its history. During the Thirty Years’ War it was used as a gunpowder store. The cannonball embedded in the façade to the right of the window is a reminder of the Prussian rampage in 1757. The rotunda is now used for religious purposes by the Vyšehrad ecclesiastical chapter.