Moated windmill

[Imperial Dave]

https://www.heritagedaily.com/2024/05/archaeologists-find-moated-medieval-windmill/151905

Well I hadn't come across this type before...

[Ian61]

Mills were certainly valuable assets so adding some protection not a bad idea.

[Imperial Dave]

Almost a defended enclosure all on its own

[DBS]

The article does offer the explanation of soil extraction for the mill's mound.  However, secondary security considerations might stil be relevant as to why excavate a ditch around the planned mound, as opposed to a single big hole close by if all you want is earth?  The downside to my mind is access - the whole point of a mill is the very regular delivery and dispatch of bulky cargo, so you would want carts or pack mules to be able to come and go easily.

[Imperial Dave]

2 birds and all that

[Keraunos]

I expect that they would have left at least one crossing over the ditch for deliveries.  From the picture you can see that because of the road, only 2/3rds of the ditch ring can be seen, and with only sections of the ditch having been excavated it would be easy for any causeway across the ditch not to have been picked up.  Unlikely there was a drawbridge.

[Erpingham]

In this case, I would go with moat meaning "surrounding ditch that fills up with water".  Looking at the photo, not only are the excavation trenches full of water but you can see the later network of field drains, so a very wet site. 

[Jim Webster]

In this case, I would go with moat meaning "surrounding ditch that fills up with water".  Looking at the photo, not only are the excavation trenches full of water but you can see the later network of field drains, so a very wet site. 

I think you've hit the nail on the head here. It was a wet area, and the builders needed the materials for a mound to set the mill on (which would both keep it from flooding and give it a bit more height to catch the wind) and the ditch would help drain the area and if needs be you could drain it into the field drains