Santa Cruz is the place to visit in Oran and the Wikipedia article gives plenty of background information.
It’s possible to walk up the hill, but be prepared for steep paths and exposure to sun and wind. Alternatively, take a taxi. The price for going up, the driver waiting and getting you down again depends on the goodwill of the taxi driver and your negotiating skills. We paid the meter price (about 800 Dinar), but people have also paid much more than that.
Unless you’re heavily interested in Spanish military architecture, the fort itself is not that exciting because it’s mostly empty halls and yards these days.
One goes up there for the views (and possibly the picnic area). You can see all of Oran, the Lion Mountains and Canastel to the east, the big salt lake to the south and more hills and the military port to the west (not photos of the latter though – the military doesn’t take kindly to that).
When we were there the church was still under reconstruction, as is the cable car which might hopefully be running again … soon. Things take time in Algeria, but they get done eventually. So, here’s to our next visit, including l’église and le téléphérique!