The Temple Dungeon plays a major role in the Priest in Peril, Nature Spirit, and In Aid of the Myreque quests. It is not very big, but holds something of value to players who have completed the In Aid of the Myreque quest. You have to have completed the Priest in Peril quest to get into the dungeon.
There are two ways in getting to the dungeon. The first one is by going east of Varrock, past the wooden gate and then north of the Temple. The second way is by taking the path west of Canifis. This is all shown in the map below.
The trapdoor entrance to the dungeon using the route from Varrock will be under a small concrete building of sorts, which is shown below.
From the Canifis route, you go across a bridge and then there's a trapdoor. That is the other side of the dungeon, and this entrance is also known as the Holy Barrier.
Starting off from the Varrock route entrance, you will find the Temple Guardian (level 30) who you have to kill during the Priest in Peril quest, therefore you won't be able to attack it after the quest.
Just past the beast, there is a gate to move on further down the dungeon.
Going past the gate and through the tunnel will be a well, which is used in the In Aid of the Myreque quest. Circling the well are some monuments. These are used during the Priest in Peril quest. The well is also used to make Rods of Ivandis: an effective weapon against Vampyres. With a rope in your backpack, use a Silvthrill Rod on the well to create a Rod of Ivandis.
Moving on past the well is a gate to Drezel.
The Nature Spirit quest starts here, as you speak to Drezel about the quest. He also plays a major role in the Priest in Peril and In Aid of the Myreque quest.
Just to the left of him in that image is a trapdoor, which leads to a library that is used in the In Aid of the Myreque quest.
Past Drezel is the Holy Barrier, which is the entrance to the Mort Myre Swamp, and the end of the dungeon.
Temple Guardian (Level 30)