Complex sentence: During the late 19th century, extensive landscaping was undertaken on the plantation; in about 1946, the plantation gardens, including the battery, were opened to the public, and remained thus until 1974.
Split into simpler sentences: During the late 19th century, extensive landscaping was undertaken on the plantation. In about 1946, the plantation gardens, including the battery, were opened to the public, and remained thus until 1974.