This is not going to be easy. First of all - you need to identify the Fortress (the easiest point, you can not miss it). Second, you will need to go on top of it. This will not be easy, but should be OK even without additional assistance. Then ... you must go inside the Fortress. Not all the way down, just 2 - 3 steps, enough to have access to the hole holding the Container. The container is close to the ceiling, you don't have to go down to the bottom. Hold the stairs well and better have a safety line with you (in case you would like to go all way down). "Climbing Gear required" mentioned on cache attributes, but this is only a safety precaution, actually the location is not so frightening or difficult to reach.