The Keys
Sir Prysin can be found in a small room on the west side of Varrock Castle (blue circle on the map). He tends to wander out of this room, however, so be warned.
At first he is surprised when you mention the Delrith.
After you explain about Gypsy Aris’ premonition, he understands the city’s plight. Unfortunately, he says that he has the Silverlight, but he needs 3 keys to open the box.
He says that Captain Rovin has a key, and tells you where he is.
Captain Rovin's Key
Climb 2 sets of stairs in the northwest corner of the castle to find him, and tell him that it's important that you speak to him. Captain Rovin is at first mistrusting of you.
After a bit of arguing however, he finally gives in and lets you have the key.
Sir Prysin's Key
Sir Prysin also said that he had a key, but dropped it down the kitchen drain.
Go back downstairs and head to the kitchen in the northeast corner of the castle. You will see a drain there.
You see the key, but you can’t reach it. You’re going to have to get it below, but you have to wash it down first.
Use the bucket of water
you should have in your inventory with the drain just outside the kitchen door to wash the key into the Varrock Sewer. If you don't have one, you can go up the stairs, and pick up a bucket
from either of the rooms, and then fill it with water from the kitchen sink.
The entrance to the sewer is just east of the castle fountain area manhole (yellow circle on the map), so climb down the manhole.
Take the red path to a small room, and the key should be on the ground, surrounded by corpses.
Just click on it and you'll get a rusty Silverlight Key.