Many years ago,many old city walls (still) existed in Beijing.Many years ago,there stood / (existed) many old city walls in Beijing.Many years ago,Beijing had many old city walls.Many years ago,there were many old city walls in Beijing____ ____ ___,there were many old city walls in Beijing看来应该是其他诸位的回答： Several years ago或者 Long time ago , you can make the choice, GOOD LUCK.