The heavy artillery introduced in England eventually spread to the rest of the British Isles. Although up to a thousand Irish soldiers who had served in Europe returned during the war, bringing with them experience of siege warfare from the Thirty Years' War in Europe, it was the arrival of Oliver Cromwell's train of siege guns in 1649 that transformed the conflict, and the fate of local castles. None of the Irish castles could withstand these Parliamentary weapons and most quickly surrendered. In 1650 Cromwell invaded Scotland and again his heavily artillery proved decisive.