Complex sentence: This new structure, later further extended, was a long open shed running parallel to two sides of the octagonal market and would also help protect the main market by serving as a breakwater to reduce the force of the ground swells and surf.
Split into simpler sentences: This new structure was a long open shed, and later further extended to run parallel to two sides of the octagonal market. It would help protect the main market by serving as a breakwater to reduce the force of the ground swells and surf.