Complex sentence: The British then occupied the fort, which was afterwards known by the name Fort Ticonderoga, and embarked on a series of improvements to the area and the construction of a fleet to conduct military operations on Lake Champlain.
Split into simpler sentences: The British then occupied the fort, which was afterwards known by the name Fort Ticonderoga. They embarked on a series of improvements to the area and began construction of a fleet to conduct military operations on Lake Champlain.