Complex sentence: With the advent of the Age of Discovery, they were considered State secrets in Portugal and Spain, very valuable in the description of Atlantic and Indian coastlines for newcomer English and Dutch raiding, and later trading, ships.
Split into simpler sentences: With the advent of the Age of Discovery, they were considered State secrets in Portugal and Spain. They were very valuable in the description of Atlantic and Indian coastlines for newcomer English and Dutch raiding, and later trading, ships.