There is an axiom that goes something like this: "Never let the truth get in the way of a good story." This could be applied to a circular metal orb embedded into the wall of a restaurant next to Edinburgh Castle Esplanade. Many tour guides will point out the cannonball to visiting tourists, and inform them that it was fired from the fortress during one of its many sieges.

While it's true that Edinburgh Castle was the scene of many battles (23 to be exact), the reality of why there is a metal sphere lodged into this stone building is a little less glamorous. This all relates to issues of plumbing and the mechanics of supplying water to the citizenry of this capital city.

Because of the aforementioned battles, Edinburgh, like many other medieval townships, erected a series of stone barricades that served as defensive impediments. Though these walls provided protection, they prevented access to an open source of potable water. City engineers had to figure out a way to allow this vital resource to continue to flow, even during times of enemy engagement.

It was decided that a reservoir was needed and that water could be pumped in using a series of pipes. The chosen location of this water supply is currently occupied by the business of the Edinburgh Woolen Mill. The "cannonball" served as a marker for the gravitational height of Comiston Hill, much like a ballcock or float valve does in a toilet. When the water went below the "cannonball," it was a sign that more water was needed to replenish the supply.

Now, this explanation is not as nearly as illustrious as some of the tales guides tell visiting punters. Like an inebriated one o'clock gunner, the cannon that is fired off every day at the castle. Or an exuberant Bonnie Prince Charlie when he was staying at Holyrood Palace. Nor an over-enthusiastic Mary Queen of Scots playing a game of skittle ball.