The evidence I used for my educated guess is that it’s put together with what looks like Portland cement that doesn't looked weathered very much. It looks like it was finished with modern day tools too. Do you see what I'm talking about? An easy way to tell how old a site might be is by taking a sample of the targets in the area.
It’s definitely a good hunting spot, but if it was really old I think the walls would have fallen down by now.