This is all good and fine having a spring supplying the moat, but the
level of the moat needs regulating otherwise the continual supply of
water would flood the entire site.

Perhaps what would be needed is some form of drainage channel, running
downhill from the site to a safe outflow such as a river, lets say.

The channel would need some form of containment, perhaps a dug channel
or just manmade levees either side.

So we need to consider looking for a structure that fulfills these
requirements, leading away from the site. A fluvial channel, like a
stream, with the characteristic erosional features along its bed that
we would expect from similar natural features.

One wonders of anyone has identified any structures that might satisfy
this notion?