Complex sentence: On January 15,1861 and January 18,1861, Slemmer refused demands for surrender from Florida militia Colonel William Henry Chase, who had designed and constructed the fort while a captain in the U.S. Army Corps of Engineers.
Split into simpler sentences: On January 15,1861 and January 18,1861, Slemmer refused surrender demands from Colonel William Henry Chase of the Florida militia. Ironically, Chase had designed and constructed the fort as captain in the U.S. Army Corps of Engineers.