Complex sentence: It was the location of the American invasion of Canada in 1812, referred to as the Battle of Queenston Heights; a commemorative sign indicates the location where the American force embarked to cross the Niagara River.
Split into simpler sentences: It was the site of the American invasion of Canada in 1812, referred to as the Battle of Queenston Heights. A commemorative sign marks the location where the American force embarked to cross the Niagara River.