How do you get Sir Prysin's key out of the drain in Runescape?

The key is stuck down a drain behind the kitchen in the castle. Go get a bucket or jug and fill it with water at any water source and the sink in the kitchen gives you water (if you did not bring a bucket, up the stairs there is a rooms at the corner and you can get a bucket in there). Use the bucket of water on the drain and the key will be washed into the sewers. Enter the Varrock sewers using the manhole located immediately east of the castle's keep. Follow the north-western path until you get to a stream and go to the small corner. Search for a rusty key on the ground; it will not appear on the minimap. Grab it.