After you have assassinated Rothbauer, exit his mansion across the main entrance and jump down into the groove from the northern side and then, down near the water. There, you will be safe from any soldiers and you can easily walk onto the first bridge without being seen. Wait for the guard by the cannon (M7,6) to go (either get him from behind or shoot; throw the body into the water), plant the explosive, dash towards the town and hide behind the barrels.
At the crossroads, go left and walk onto the road. Go west. You only have to be more careful by the second bridge, where there are several guards. You do not have to fight them. Follow the tunnel, towards the second cannon (M7,7) on the hill.
It is being guarded by three soldiers. You will either have to distract them or eliminate them, where the latter is safer this time around, and more convenient. There is nobody around who would come to their aid.
Go north and onto the wooden bridge. You will reach a tunnel, where there is a bunker entrance (M7,8).
Inside, there are no more than two enemies: one patrolling the corridors and the other one in the room with the safe in it. You can either blow up the safe (M7,9) , or use the combination that you have found on Rothbauer. Return to the road now head north, towards the town.