Complex sentence: Lt Slemmer knew that Fort Pickens was easier to defend, so he spiked the guns at Barrancas, loaded ammunition and supplies on a flatboat, and moved his company across the bay to Fort Pickens which was held by the Union throughout the Civil War.
Split into simpler sentences: LT Slemmer knew that Fort Pickens was easier to defend, so he spiked the guns at Barrancas and loaded ammunition and supplies on a flatboat. He moved his company across the bay to Fort Pickens, and held that fort for the Union throughout the Civil War.