Just past the depleted ore mine there is a barrier you will not be able to get past until after getting the battle tower.
You may want to leave enough opponents here in order to make sure you can level again after finishing off Broken Valley. At the end of Maxos Temple you get access to a teleporter which will bring you to a new area. Your first encounter there you can mindread to bring you up to the next level, so if you make sure you level just before going there, you can get a full level worth of experience.
Playing a ranger in DKS (on nightmare) I went to the fjords at level 13, got up to 16 fairly quickly and decided to go back and finish off a couple places in Broken Valley and head to Maxos temple (reaching level 17 there). Back to the fjords, clearing the beach by the waterfall and swimming over to the one behind the anti-dragon shields brought me to level 18, clearing past the depleted ore mine brought me to 19, and which point I headed back to Broken Valley long enough to kill a rabbit and use Belegar's telleporters (the 2 that need a password).
After that, I saved at each level so I could reload when I got as far as I could go, which was level 22 and a half. I reloaded my 'just turned level 21' save, turned in a selection of quests trying to just reach level 22, and went through the bandit camp to get the last 28 experience points I needed.
Before using the final teleporter in Maxos temple is a great time to do any outstanding mindreads, as any debt will be wiped out your first mindread in the new location. In my case there were only a couple NPCs left to mindread.
In the PC version there should be a dialog box to the right of the UI at the bottom of the screen if the crosshairs in the middle of the screen are pointed at an NPC or object that can be interacted with (door, chest, etc).