Complex sentence: He built and remodelled a number of country houses, including Thirlestane, Hopetoun, and his own house at Kinross, although his most significant commission remained the rebuilding of the Royal Palace of Holyroodhouse in the 1670s, which gave the palace its present appearance.
Split into simpler sentences: He built and remodelled a number of country houses, including Thirlestane, Hopetoun, and his own house at Kinross. However, his most significant commission was the rebuilding of the Royal Palace of Holyroodhouse in the 1670s, which gave the palace its present appearance.