I'd added two clips on Korean drama about the Battle of Ansi Fort in attempt to find more about early part of DJY that I missed. DJY's mother was pregnant with her son while Ansi Fort was undersiege by the Tang Chinese (I'm not sure if his father was commander of that fort at the time, but that amusing enemy, peasant turned general Xue Rengui was there in Tang army).
From comments made much later in drama, it may have been DJY's mother who got idea for undermining hill that Chinese army built to over look the fort in order to attack from above. If it wasn't her idea, she helped dig tunnels which were flooded (by diverting river?), causing collapse of hill, wiping out much of Chinese forces (maybe a million? Fairytale like exageration, perhaps!)