Complex sentence: Its original purpose was to exploit the gold fields up the Gambia River identified by Prince Rupert during the Interregnum, and it was set up once Charles II gained the English throne in the Restoration of 1660.
Split into simpler sentences: Its original purpose was to exploit the gold fields up the Gambia River, which were identified by Prince Rupert during the Interregnum. It was established after Charles II gained the English throne in the Restoration of 1660.