Prison is the opposite of Cell. Cell is related to Part. Part is related to Sleeve. 
Vulnerable is similar to Weak. Weak is a synonym of Accessible. Accessible is similar to Available. Available is derived from Ability. 
Prison <--Antonym--> Cell <--RelatedTo--> Part <--RelatedTo--> Sleeve 
Vulnerable <--SimilarTo--> Weak <--Synonym--> Accessible <--SimilarTo--> Available --DerivedFrom--> Ability 