While the current castle, which houses the Inverness Sheriff Court, was only constructed in 1836, a castle has stood on this spot since the 11th century. Most of the structure is not open to the public, but visitors can get a great view from the north tower. The first structure here was built by Macbeth (yes, THE Macbeth), then later it was razed by Mael Coluim III and a new one erected. Basically the process continued several times.
23 miles long and deep enough to nearly submerge the Empire State Building, Loch Ness is the center of legends and mysteries stretching back centuries. Most notable among these, of course, is that of the Loch Ness Monster, but to be sure there are other tales to tell as well.
As for explanations of what the creature might be, there are some remote theories beyond logs and lies. For instance, Bottlenose dolphins and Harbor porpoises have never been recorded in the Loch itself, but they have been spotted in the River Ness, so it might be possible that errant specimens could attest to at least some sightings of the oft-reported creature. As I have talked about previously, the a pod of such animals breaking the surface in the typical arch-backed manner of mammals could explain the "humps" many witnesses have reported seeing in the water.
Another suspect in the case of Nessie might be sightings of the critically endangered European Sea Sturgeon. This rare fish can, in extreme specimens, reach upwards of 20 feet in length and weigh as much as 800 lbs. Could it have been something like these animals that caused the tragic death of John Cobb who fatally attempted a water speed record in 1952 when his boat, Crusader, struck an unexplained wake? Or is it less of a stretch to think that some fantastic, unrecognized animal does lurk within this storied lake's murky depths?
It's also believed that Spanish gold funded the Jacobite Army as it fought to hold the castle. After the defeat of Culloden in 1745, 7 crates of the gold were hidden away. Some say it lies in the nearby forests while others are convinced it remains hidden somewhere either in castle or in the Loch itself.
There are Templar tales loosely tied to the castle as well, such as the treasure of King Baldwin II after his death in 1131. Much of it is reputed to have been taken to Rosslyn Chapel near Edinburgh, but some may have found its way to Urquhart.