The current fortress is situated on the site where Belgrade was founded during the Roman era in the 1st century AD and on which it developed right down to the second half of the 18th century. The fortress we see today had its walls reinforced around the mid-18th century--the red bricks date from the brief period of the Austrian occupation and the gray stonework is Ottoman. The walls of the fortress encircle the old 15th century Acropolis or the Upper Town, when Belgrade was proclaimed the Serbian capital in 1404.