"Fallen-in" is an adjective used to describe something that has collapsed, sunk, or caved in. Some antonyms for this word could include steady, secure, fixed, strong, robust, and sound. These words imply a sense of stability and fortitude, suggesting that the opposite of something that has fallen-in is something that is steadfast and unyielding. Additionally, phrases such as "built up" or "reinforced" could also serve as antonyms to fallen-in, indicating that something has been strengthened or constructed to prevent a collapse. Overall, the antonyms for fallen-in signify the opposite of instability and vulnerability, indicating a sense of strength and reliability.