The fortress is shaped like a long stone star, with thick walls, towers and stone gates. Inside, instead of soldiers, you find ice-cream sellers, chess players, a tiny zoo and a small dinosaur park. The grass between the walls is full of picnic blankets in summer.
From the top, you can see far across two rivers - and the city stretches in every other direction. Locals say it is the best place in Belgrade to watch a sunset. On a clear evening, the sky turns orange and pink behind the bridges.
Bits of the fortress come from many different times. Some stones are Roman, some are medieval, some are from the Ottoman period, and some were added by the Austrians. Like a layer cake of history, you can walk along walls built by builders thousands of years apart.
Children especially love the cannons. Old metal cannons are lined up along the walls, pointing out over the rivers. You can sit on them, but they haven't been fired in a very long time - they are just decorations now.