Three Paladins are protecting the treasures upstairs. They must be fought or blocked behind Doors in order to go up. Rapidly clicking the stairs will also work, similarly to how rapidly clicking Thieving Stalls will work before the guard initiates combat.
Upstairs is a door that requires 61 Thieving to pick lock and gives 50 Thieving experience. It doesn't require a lockpick, though. The door leads to a tiny room with multiple Paladins, very useful for Thieving training. Furthermore, there is a ladder to top floor with level 72 Thieving chest that gives 500 Thieving experience, 1000 Coins, Raw Shark, Adamantite ore and Uncut sapphire every time thieved. Thieving the chest will trigger a trap that teleports the thief to Witchaven dungeon entrance, east of Ardougne.