The Hall of Echoes is the end of the game, so you'll want to finish off everything in Broken Valley, Sentinel Island and the fjords before entering.
The cutscene at the mine I was referring to was in Broken Valley, before getting the battle tower, near the quarry waypoint shrine, not far from the large glowing 'hellgate' orb (with the book on a pedestal that you could read to trigger the appearance of a series of opponents), a little south of the bandit camp entrance. After the cutscene Talana tells you not to enter because you are not strong enough, but the opponents outside are easy enough to deal with. Entering the tower early, though, there is a jump in the level of the opponents (you may be there level 12 or 13 the first time you go by, but the guards inside are something like level 24.
tnx for telling me this 