Complex sentence: By the close of the 14th century Dunnotar was firmly under Scottish control; moreover, rebuilding continued by William Keith and his successors including the fifth Earl, George, who founded Marischal College in Aberdeen.
Split into simpler sentences: By the close of the 14th century Dunnottar was firmly under Scottish control. Rebuilding was carried out under the Earls Marischal, notably William Keith and his successors including the 5th Earl, George, founder of Marischal College in Aberdeen.