you can find the key in the drain by the Yews by Varrock Castle. use a bucket of water on the drain, go underground (by Varrock Castle). there are two ways, take the left way and go the first time you can go left, then go left and there is a rusty key. (for extra: the last two keys are upstairs in Varrock Castle and by The wizard Tower. 1. talk to the guy on the 3rd or 2nd floor ( i don't know who and what floor.). make a good story with the guy and he gives you the key.
2. take 25 normal sets of bones to Wizard Tower. talk to Traiborn, ask him to a key that sir Prysin has given him, but he doesn't know what are you talking about. but he remember an another key. he ask you to give him 25 sets of bones. get these sets and give it to him. than he gives you a key, the third key.
Sir Prysin will give it to you after you talk to him.
Assuming that you used the bucket of water with the drain, the key should be here in the mud http://img502.imageshack.us/img502/6241/varrocksewers.jpg
In future can you please give us more detail on what your after, there is more than one key in Runescape, thankyou.My Guess:You are doing the "Demon Slayer" quest, if I'm right to find the first key go to the northwest corner of the castle to find a set of stairs. Climb them twice to reach a room with Captain Rovin. Speak to him and tell him that a demon is going to invade the city, that you have to defeat it, and you need his Silverlight key. Select the option that says "Sir Prysin said you would give me the key" and then select the option that says "Then why did he give you the key?" After bringing up Sir Prysin's clumsiness he will hand it to you.
It's Varrock Castle. To get it you need to get a bucket of water and use the bucket of water on the kitchen drain. Thie key will fall into the sewer, you'll need to go to the Varrock Sewers to pick up the key. The key will be on top of a pile of mud near a clump of several skeletons.
You kill chaos Dwarves there I think, and they drop the key.
It is just outside the inner castle wall, by the kitchen. The kitchen is in the north-eastern portion of the castle.
If it's to do with Dungeoneering, Find a door with the matching shape and colour.
The key card is dropped at the end of the scooter chase. It is used much later in the game.
sorry you cant make keys on runescape unless of corse it is the crystsal key.
There isn't a key for it unfortunately.
I am sorry, but you can not actually answer this question, because there are many tunnels in RuneScape. Especially for members there are a lot. There are also different keys, for different places and different quests.
A brass key.