Ardougne Castle is a castle in East Ardougne near the border to West Ardougne. King Lathas was king of Kandarin until Plague's End; after this quest transpires he will be replaced by his cousin, King Thoros, who resides on the 1st floor[UK]. Players can start the quest Underground Pass here. Aside from the king, many Paladins wander the area around it.
There is a chest that gives you 1,000 coins, a shark, adamantite ore and sapphire when you picklock it. Bear in mind that you need to picklock the door as well. There are two both on the same floor. It requires 72 thieving.