This way you entrer the place behind Natinal theatre (theatre being to the left from this view). You can go to Laterna Magika - another interesting theatre - from here. Or you can take your skateboard and join the others who skate at this place. And if you're lucky, you wouldn't end up under the tram going on Narodni trida street, which is on the opposite side (over the red lighted building in the photo).
See older posts with National theatre
Official sites of:
National theatre
Laterna Magika