Go downstairs and kill the level 30 Monks of Zamorak until you get a Golden Key. Now, go back outside, down the trap door where the Temple Guardian was found, and through the previously locked gate. You will find a room filled with a circle of Monuments. Study the monuments until you find one that says "Saradomin is the key that unlocks the mysteries of life,"and use the key with it to get an Iron Key. Use this to unlock the cell door.
You must go back to where you killed the monks where the altar is and you have to kill them until you receive another golden key. You cant get another unless you've lost the key.
after you do priest in peril quest, you pass through a magic barrier in the basement of the temple. you can then run there easily.
If you are a member and have done preist in peril quest, I recommend you train it on Experiments under the grave stone near canifs, they are great for training on they are only level 25 and they have 100 HP, meaning around 400 Exp per kill.
You will need to complete the "priest in Peril" quest.
From Peril to Peril - 1914 was released on: USA: 17 October 1914
Go north east from varrock, and you need to do priest in peril.
The Peril - 1912 was released on: USA: 30 May 1912
Canfis slayer tower, Canfis is a town reached by doing the priest in peril quest.
On the Line of Peril - 1912 was released on: USA: 31 October 1912
The Peril of the Plains - 1911 was released on: USA: 27 October 1911
The Cowpuncher's Peril - 1916 was released on: USA: 17 June 1916
In Peril of His Life - 1913 was released on: USA: 29 November 1913