Jamieson, Fausset and Brown on 2 Samuel 5:6

6. the king and his men went to Jerusalem unto the Jebusites--The first expedition of David, as king of the whole country, was directed against this place, which had hitherto remained in the hands of the natives. It was strongly fortified and deemed so impregnable that the blind and lame were sent to man the battlements, in derisive mockery of the Hebrew king's attack, and to shout, "David cannot come in hither." To understand the full meaning and force of this insulting taunt, it is necessary to bear in mind the depth and steepness of the valley of Gihon, and the lofty walls of the ancient Canaanitish fortress.