Complex sentence: The fort was constructed at a turn in the lake at its narrowest point, the cannons of Fort St. Frédéric and later the Crown Point fort were capable of cutting off all travel north and south on the lake.
Split into simpler sentences: The fort was constructed at a turn in the lake at its narrowest point. The cannons of Fort St. Frédéric and the later Crown Point fort were capable of cutting off all travel north and south on the lake.