Enjoying Sardinia

Sardinia doesn't have much to see but has a lot of awesome roads to ride, most of them in very good condition and with almost no traffic to care about. 

Can someone explain me what this traffic sign is supposed to mean?

While enjoying the good tarmac I saw a sign about some roman historic site and decided to take a look and follow the signs and ended up here.

And at the end four closed gates and nothing to see...

Best enduro ever :-)