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; the sink in the kitchen is close and convenient (if you did not bring a bucket, up the stairs there is a room right behind you with a bucket and theres a bucket in front of the drain just outside the kitchen). 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 search around there. It is just beside the skeletons under a pile of mud. Search for a rusty key on the ground; it will not appear on the minimap. Grab it.
Use this site, it teaches all about how to get the keys. www.tip.it/runescape/index?php?rs2quest_id=2
bucket of water
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
get a bucket of water pour it down the drain and get it in the sewer go to runehq.com or runescape.wikia.com for quest guides
in the drain in varrock place us water in the drainage pipe and then go to the swears to wear the pipe ends
just head down, into the sewer, now just follow the hallway north. Go to your left, and you will see a little glint where the key is, if you already don't see it.
use a bucket of water on the drain
sorry you cant make keys on runescape unless of corse it is the crystsal key.
Right click the water then select use then click the drain.
There isn't a key for it unfortunately.
A brass key.
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.