Complex sentence: The Golden Gate, originally a tower over the city's main gate, built in 1158-64 but grossly reconsructed in the late 18th century, to prevent the delapidated structure from tumbling down.
Split into simpler sentences: The Golden Gate, originally a tower over the city's main gate, was built in 1158-64. The gate acquired its present form after having been grossly reconstructed in the late 18th century, to prevent the delapidated structure from tumbling down.