This dungeon, used in the Observatory Quest, it is located southwest of Ardougne under the Observatory. The poison spiders in this dungeon originate from the chests. If a chest is opened there is a chance that a poison spider will appear. The key you will need for the quest is in a chest located in the south eastern most corner of the dungeon. This key will open the gate guarded by the 'Goblin Guard'.

Goblin (Level 2 and 5) Goblin Guard (Level 42) Poison Spider (Level 31)