Image of Maiden Castle (Dorchester) (Hillfort) by postman

One thing I wanted to see was the Roman temple ruin, I got all the way round and hadn’t found it so I went a looking, having not much idea what I was looking for I thought this was it, even though I had a faint recollection of there being stone work. Clearly this strange square feature is not the temple I was looking for, but what exactly is it?

Image credit: Chris Bickerton