Roman Key is an unofficial sidequest which can be started by talking to wounded man in the Swamp area. It is given this name, since acquiring the key and using it is the main goal of the quest.
South of the tower in the middle of the swamp, you can find Leofaeg Hydebaldessunu who was recently attacked and robbed by a large group of robbers. He acquired the key from someone who told him that it could be used to obtain the weapon from a Roman hero which was hidden somewhere inside the tower. He gives you all the information about the key and where the group is staying.
Though it might be possible to heal the man if the player character has a certain rank in the healing skill, it should be higher than rank 2.
If you have Roskva The Black in your party, you can ask her to try and heal the man. You will gain 5 reputation with Northumbria. It is not confirmed if simply a herd member with a healing skill of more than two will always work.
South in the area, you can find the group of eleven men and women in the same place as the menhir from The Isle of Apples. Talk to the leader about the robbed man and a battle will start. With 4 diplomacy, you can convince two men that their group of poorly trained and armed troops will lose against you, and then they will flee before the eight remaining men and their leader draw their weapons.
After the battle, you can loot the key from the leader's corpse and then head towards the tower. If the tower isn't visited yet and the quest The Mercian Connection is active, you have to go through some dialogue, but the weapon can still be acquired before meeting the emmisaries of Mercia and Osred. The weapon can be found by clicking on one of the bricks in the northern wall. In case you are struggling to find it, you can press the Alt key in order to have the spot encircled by a white line.
The dagger Carnwennan is the reward.