Complex sentence: It is a 4 storey concrete building with of armour on the seaward side, and originally armed with four 6-inch guns, built with great difficulty as its sandbank is below low water.
Split into simpler sentences: It is a 4-storey concrete building with of armour on the seaward side, and originally armed with four 6-inch guns. It was built with great difficulty as its sandbank is below low water.