Complex sentence: It commemorates over 100 Covenanters who died on the gallows between 1661 and 1688 during the The Killing Time, prompting the snide remark by one of their opponents that they had chosen "to glorify God in the Grassmarket".
Split into simpler sentences: It commemorates over 100 Covenanters who died on the gallows between 1661 and 1688 during the period known as The Killing Time. One mass execution of prisoners prompted the snide remark by an opponent that they had chosen "to glorify God in the Grassmarket".