The Zollverein site as such can be visited free of charge. Some buildings are currently used as museum, restaurant, ... and can be visited in this capacity. However, if you want access to (some of) the old production buildings, a guided tour is the only way.

We took the 4 (!) hour 'Von Kohle und Koks' tour which covers the both shaft XII coal mine and the coking plant. Unfortunately, this tour is only available in German but other (shorter) tours are also offered in English.

We especially liked the coking plant as this part seemed a little less 'cleaned up' than the coal mine. Apart from the tour, it is also worth wandering around the rest of the site (shaft 1/2/8, the old train tracks, ...).

Given the size of the site, one can spend a full day in Zollverein, especially if you intend to visit the musea (which we did not do).

As mentioned by other reviewers, Völklingen Ironworks comes to mind as comparable site (i.e. closed down heavy industries). We preferred Völklingen over Zollverein, but Zollverein is certainly also worth a visit if you like 'industry WHS'.