Why do you use a sledgehammer anyway.
Not very stealthy...
It's small, like a short 1-handed maul really. My dad uses it in combination with the axe to cut wood.
He hits the wood as hard and accurately as he can with the axe then hits it with the hammer to break it up instead of hitting it with the axe repeatedly, with the risk he won't hit the same spot.
It's used for a public assassination or when you need to make a statement of the victim.
Mostly used when a client wants the murder to be quickly discovered.
My favourite part about Assassin's Creed is that every assassination is a public one, in front of about 2000 people. I thought "assassinate" meant stealthily kill someone.