Simplify the following text for a Grade 6 native English-speaking person.
Original: Often referred to as The Rock , the small island early-on served as a lighthouse , a military fortification , a military prison , and a Federal Bureau of Prisons federal prison until 1963 .
Simplified: It was first a lighthouse , then a military fort , then a military prison followed by a federal prison . In 1963 , it became a museum .