The Imperial Legion quest Properly Purloined sends the player to guard the sewer entrance to the Ebon Tower's Treasure Chamber. It is a low level quest where you end up killing a couple of rats and dealing with a thief. However, the key the player is given to access the sewers allows them to easily access the treasure chamber and grab some very nice loot, such as the Cuirass of Cassynder Septim, with little effort and no consequence.

An additional layer of security should be added to prevent this from being so easy. An extra gate that the Ebon Towers Sewers Key doesn't open, remove the existing gate from being opened by the key, have the guard already down there challenge the player if they try to pass, have the hidden door's activator be hidden somewhere, etc. As it is right now, a level 1 player can grab some ebony-tier gear, one with a nice +1HP/-1Magicka CE enchantment.

Cells:
Ebon Tower, Palace: Treasure Chamber
Ebon Tower, Sewers: Upper

Ebon Towers Sewers Key:
TR_m3_q_IL_sewer_key