Jerusalem is the inner city of God, and David is the fulfilled self rising up within a person to claim it. The Jebusites who already hold the land are the settled convictions of the senses, the beliefs that insist things cannot change. Their taunt, that the blind and the lame will keep David out, is the old certainty that what you cannot see and cannot yet act upon has no power to enter.
The blind and the lame are that unbelief: eyes that see no evidence and limbs too weak to move toward the wish. They must be taken away, which is to say the assumption of impossibility is set aside before the city opens. David enters not by force against the walls but by the inner shaft, the quiet climb from outer fact up into imagined fulfillment. The fulfilled state is reached from within, never stormed from without.