Complex sentence: Spitbank is smaller than the two main Solent forts, Horse Sand Fort and No Mans Land Fort, and its main purpose was as a further line of defence for ships that made it past the two main forts.
Split into simpler sentences: Spitbank is smaller than the two main Solent forts, Horse Sand Fort and No Mans Land Fort. Its main purpose was as a further line of defence for ships that made it past the two main forts.