The highest estimates I've been able to find are about 12-14 meters for the Theodosian Walls in Constantinople, tied with a couple other places in China and India. Maybe fantasy and artistic license have led me astray, but that seems a bit on the small side to be the largest of all time.
Are there any extant or demolished fortifications from before about 1815 that were higher? Towers and keeps not withstanding, I'm mostly interested in the height of the outer defensive walls.
Sections of Dubrovnik (Croatia) walls are 25 meters tall. I don't know if there's anything taller.
Do you really think 14 meters is that small?
Well, AFAIK, Mehrangarh Forts's walls are 36 m high. There ought to be taller ones. Try getting information about forts with high walls, Chittorgarh fort and Kumbalgarh fort (this one has a long continuous wall (38km))come to mind.