Heritage Open Day: A guided tour exploring the Colchester places and people at the time of the English Civil War Siege in 1648. In 1648, King Charles I was being held prisoner by Parliament and a Royalist army seized Colchester as part of a concerted attempt throughout England to gain his freedom. The resulting siege lasted for eleven weeks with great hardship for both the Royalist army and the citizens of Colchester. Following the Royalist surrender, the cavaliers Sir Charles Lucas and Sir George Lisle were executed at Colchester Castle.