One must keep in mind that David was the second king, Hezekiah was an hereditary monarch in the line of David, and he reigned centuries later. This leaves us with the little problem of explaining how David could have invaded Jerusalem via a water gate that would not be constructed until several centuries later. (Perhaps he morphed through solid rock). This is also brings up interesting questions about the dating of some of these fabulous David stories. Some of these David fables appear to have been invented many, many centuries after the times they purport to describe. This is fine, if we understand that we are dealing with fables, and only becomes a problem when people start insisting that such fables are 'history,' and 'infallibly inerrant history' at that.
The stories about David are rife with these sorts of internal inconsistencies and conflicts, often within a single paragraph. Analyzing these stories reveals something of the editorial process involved in manufacturing the Bible. It was obviously spun together like a woven clothe from many different colors of thread.
To illustrate this point I will bring to your attention a couple of very interesting passages from the books of Samuel which tell of 'the history of King David,' of Samuel and King Saul.
"The Philistines were subdued and no longer invaded Israel as long as Samuel lived for the hand of YAHWEH was against them...peace was maintained." (1 Samuel 7:13) In chapter 13, "The Philistines gathered together to fight against Israel." (1 Samuel 13:5) "Samuel left Gilgal and went his way" (1 Samuel 13:15) . "The Philistines gathered for war at Micmash." (1 Samuel 13:11) "No blacksmith was to be found in all Israel, for the Philist ...