The stairway to Trsat is in Rijeka's center and contains a total of around 600 steps. It will take you from Rijeka's low point to its highest point, the keep at the top of the hill which oversees the city.
Milengrad is a medieval fortress on top of the Ivanscica mountain in north Croatia. It's a medium intensity climb of two hours or so. The mountain contains several medieval ruins like these, but this one is by far the largest.
This is the park called "Perivoj Zrinskih" in Cakovec, Croatia. It was home to the Zrinski castle long ago (see map), and the depression you see here was filled with water once. Today, it's a beautiful park in the center of town.