It might be easy to dismiss this as a historical curio, but it's actually not too bad. This is a historical romance/tragedy about a captured king brought into slavery in English Suriname. 
 Behn's attitude toward slavery and monarchy is puzzling to us more modern readers. She's still an avowed pro-monarchist Tory, and it's as easy to interpret this as a pro-Stuart piece as an anti-slavery one. Well, humanizing some black people is a good start for the 17th century, I suppose.