The term 'Solidly Based' can be represented by two crossword solutions: FOURSQUARE (10 letters) and TENABLE (7 letters). FOURSQUARE is an adjective that means firmly established or securely built. It is often used to describe a building or structure that is strong and stable. TENABLE is an adjective that means capable of being defended or maintained. It is often used to describe an argument, opinion, or position that is well-supported and can withstand criticism. Together, these two words suggest that something is strongly established and can be defended.