You are right, the lever appears broken as I checked it in Siege Editor. Incidentally activating the lever should also spawn an ambush. Its possible that defeating the ambush is meant to activate the secret door but it's not working in DS2. Again DS2 also has issues with the placements of DS1 Levers and Use Points so I've sent you a private message with all three fixes. No guarantee of success as I can't test it.
Also sorry to deflate your hopes but that lever just hides a secret area. You can't progress to the next region from there. Checking stitch_helper.gas found in the editor folder of the region reveals that d_a_k has only one region attached to it. Checking Winterdorf reveals that it's linked to three regions. D_A_K which is a dead end, bergpass and D_K_H.
Looking at the stitch_helper.gas for each region reveals that you're looking for bergpass. That region also has two other regions attached to it, D_V_G which is a presumeably dead end and D_G_H which connects to Himmelpfort. From there its linear through 3 more regions until you get to the final region Der_Alte_Wer just pass the town of Durathan.
There's one small region, reisezentrum, that isn't linked directly to any other regions, it seems you need the hub portal to get there according to the elevator.gas.
The fix for the secret door in D_A_K actually leads to another lever which allows you to walk through a wall up a flight of stairs through regions D_G_H, D_V_G and Bergpass all the way to Himmelpfort. There are a lot of nasty bosses and traps where you can fall through the snow in Bergpass but you can get there if you work at it.
Thanks for fixing the secret door Iryan because I was able to locate two of the save points I installed but could never find before because of the broken lever.
The screen shots are a bit out of order as these were taken over several trips do to getting killed or forgetting to pick up health potions on another map so I could Mark my way.
The fix for the secret door in D_A_K actually leads to another lever which allows you to walk through a wall up a flight of stairs through regions D_G_H, D_V_G and Bergpass all the way to Himmelpfort. There are a lot of nasty bosses and traps where you can fall through the snow in Bergpass but you can get there if you work at it.
Thanks for fixing the secret door Iryan because I was able to locate two of the save points I installed but could never find before because of the broken lever.
The screen shots are a bit out of order as these were taken over several trips do to getting killed or forgetting to pick up health potions on another map so I could Mark my way.