There is a pedestrian tunnel in Zagreb, it is called the Gric tunnel because it is underneath the Gric neighborhood, in the Upper Town.
In was built during World War II, in 1943-1944, and was meant to be a shelter for the civilians due to the frequent bombings of the City, and it was also used during the war of Independance. It was renovated in 2016. Nowadays it is used for exhibitions and events, and for us to walk through, of course. And you know what? It’s super fresh there, so if you are there during the summer, it’s much better to walk underground, to avoid the heat above ground. It seems it is decorated during Christmas time! so it would be worth experiencing that, but I was there in February before the global pandemic so I did not see that.
There are several entrances, and it’s open from 9a.m. to 9p.m. Always make sure to check updated information from the Zagreb tourism office before going.