Complex sentence: With Sluis gone so too had the benefit of a good haven for Spanish gallies a conveniently situated place for an invasion of the United Provinces which would have enabled the Spanish to disrupt Dutch trade.
Split into simpler sentences: With Sluis gone so too had the benefit of a good haven for Spanish gallies. A conveniently situated place for an invasion of the United Provinces which would have enabled the Spanish to disrupt Dutch trade.