Complex sentence: Fort Trumbull on the New London side was little more than a redoubt open on the inland side, while Fort Griswold in Groton was a more substantial, roughly square, bastioned stone fort surrounded by a ditch and some outer earthen defenses.
Split into simpler sentences: Fort Trumbull on the New London side was little more than a redoubt open on the inland side, while Fort Griswold in Groton was a more substantial fort. It was roughly square and bastioned, surrounded by a ditch and some outer earthen defenses.