Ancient fortress at the mouth of Djerdap Gorge, Serbian side of the river Danube. Windmills on the other side of the river belong to Romania:

For more details see here…

https://steemit.com/landscapephotography/@lighteye/golubac-eng-srp-golubac

https://steemit.com/architecturalphotography/@lighteye/3i6yj4-golubac-fortress-eng-srp-golubachki-grad

…and here:

https://steemit.com/travel/@lighteye/golubac-fortress-eng-srp-golubachki-grad