The Burthorpe's Imperial Guard is fighting an uphill battle against invading trolls. Captain Denulth has plans to turn things around, but needs your help. An alternate route up the mountain must be found and scouted, but before that he has some other concerns…
Speak to Denulth, located in one of the tents in the Burthorpe guard camp. Ask him “Do you have any quests for me?” and answer “No, but perhaps I could try to find one?” Before finding a path up Death Plateau, Denulth wants you to to recover the combination to the castle's storage room.
To track down the guard in charge of the storeroom, first speak to Eohric upstairs in Burthorpe castle. Tell him “I'm looking for the guard that was on the last night.” He will tell you where to find Harold, the guard who was on duty. Go to the bar nearby in Burthorpe, be sure to buy an Asgarnian Ale if you don't have one already. Speak to Harold upstairs in his room, but he will refuse to say anything. Return to Eohric, and he will tell you Harold might be more cooperative after you give him a drink. Talk to Harold again with an Asgarnian Ale then talk to him again. Talking to him about the combination won't really yield any results. At this point, if you have a premade or player-made Blurberry special, ask Harold about another drink.
After giving him a Blurberry special (or not), ask him “Would you like to gamble?”. You'll be brought to a dice game where you can bet between 60 and 1000 coins for double if you win. The goal is to make Harold run out of money. If you gave him a Blurberry special, the dice game is rigged in your favor: no matter what you roll, it will be considered a win. Simply bet several hundred gp or more, and Harold will be broke. If you didn't give him a Blurberry special, an effective method is to start a bet of 100gp and double it every time you lose, while resetting the bet to 100 after a win. Either way, after losing his couple hundred starting gp in addition to whatever he won off of you, Harold will give you an IOU. Read the IOU, and you'll find that it's the combination you're looking for. Keep this combination in your inventory for the rest of the quest, or you'll have to get another from Harold at the end.
With the combination in hand, go to Burthorpe castle, pick up the stone balls on the floor and use them on the table so that they form the pattern described in the note. Pictured is the correct arrangement.
When you place the last ball in the right position, you'll get a message that the door has unlocked. You're now done here.
Head northwest out of Burthorpe, until you see a cave entrance (roughly north of the Warrior's Guild).
Inside, talk to Saba. Ask him “Do you know of another way up Death Plateau?” Leave and continue west, however, don't go north of where you see a wounded solider on the ground, instead, head further west and south onto a winding path. Try to enter the door of the house here, once inside talk to Tenzig and agree to help him. He'll give you his pair of Climbing boots.
Head back to Burthorpe and find Dunstan, whose house is in the northeast part of town where the anvil icon is. Talk to him, and he'll agree to repair the boots if you bring him an iron bar and a certificate from the Imperial Guard regarding his son. Go and talk to Denulth again, who will give you a Certificate. With this, an iron bar, and the climbing boots, return to Dunstan. Talk to him again, and you'll eventually receive Tenzing's Spiked boots. With these as well as 10 trout and 10 bread (both unnoted), return to Tenzig, and he'll give you a Secret way map to Death Plateau. Keep this for the rest of the quest.
Go out the other side of Tenzig's house, over the stile on the path leading north. Continue along (you do not need to climb over any rocks you see), until you reach near the end of the traversible path. Eventually, a message will appear saying that you've successfully scouted the secret path (picture below).
You may have to run back and forth near the pictured area a couple times to trigger this. After this, return to Denulth with the Secret way map and Combination.