[quote=Guilty Spark]
Point taken.A mild counter-point, it was less weathering and more warfare that eroded their fortress. After several take overs most of the original stone work was destroyed. In most cases in which they weren't constructed under severe conditions, however, Old Kingdom stonework was preternaturally resilient. A lake forming over it, though, would be effective weathering.
[/quote]

What if there was an unknown Old Kingdom city/town in the depths of the Lake? Some kind of Relic-related or magic-related or whatever else the Relics run on end up weakening the ground below the city, which created a large, uneven crater that filled up with water, whether from weather or more Relic-related incidents.

Just a suggestion.