The antonyms for the word "left ruins" would be words/phrases that describe conditions that imply improvement or progress. Some of the antonyms could include "built up," "restored," "repaired," "thriving," "flourishing," "prosperous," and "developed." If something is built up, it means that it has been constructed or expanded upon, indicating progress. Restored or repaired means that something damaged has been fixed and returned to its original state, suggesting improvement. Thriving, flourishing or prosperous signify growth, whereas developed means something has been advanced or enhanced. All these antonyms convey a sense of positivity and progress, which are the opposite of the meaning of "left ruins.