In the bastion select Battle > Dungeons.  Spiders Den is on the map here. 

Welcome to the game!

Have you tried asking your clan?

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When you open the dungeons section, it will show you centered in the potion keeps, scroll right to see the loot dungeons

BTW, I would like to add to the conversation. Would the OP like to post a screenshot of the quest that they picked? There are 3 level of quests to choose from; from Basic, Expert, to Elite. As a new player, you should be picking quests from the Basic list. While the rewards are better for Elite, to be able to achieve and complete the quest takes a team of high value.

Basically, there are 2 reasons NOT to choose from the Elite list yet. First, you are unable to complete the quest and end up using 1 of your 3 choices to earn nothing. Second, if you're in a mid-level clan, you may take a quest away from someone else in your clan that would be able to finish and complete it. Some times people get mad if they pick a quest a level down if they see someone take a quest they could have done and see that someone didn't complete it. As a new player, there are a few Expert quests that you can probably do if you see them available. A few that come to mind are "Win 200 multi-battle attempts", "Sell 2 mill in silver of equipment" and "Get the first turn in classic arena 30 times (and win)". Those three are fairly easy to do even at level 20 but can be a fight to take them if you're in a low level clan. If I saw all 3 available as a new player, I would only take 1 of them unless you have players in your clan that are above level 50 that say you can take more. It's just proper etiquette for clan members IMHO.