2 Samuel 5:9

9. Built round about from Millo and inward Millo was a fortress or rampart of ancient Zion, apparently so called from having been filled in ( מלא ) with earth and stones. The article connected with the Hebrew word serves to designate it as some well known fortress, and it was evidently a noted stronghold before the city was occupied by David. It was afterwards built up again and strengthened by Solomon, (1 Kings 9:15; 1Ki 9:24 ; 1 Kings 11:27,) and later still by Hezekiah. 2 Chronicles 32:5. It seems to have been situated on the northern side of Zion, where the natural defences were less strong than on the other sides; and from it, as a bulwark, David built all round the northern side a strong wall, and fortified it inward towards the south, so as to make it more secure against assault than it had ever been before.