The way I beat the level:
Sneak up on the postman, anesthetize him, and take his goodies.  Sprint over and hide your guns in the delivery boy’s boxes.  Sprint back around to the front and go through the front gate.  Walk through the front door.  When the maid takes the flowers from you, stand still.  Shortly, the guard there will follow her through the double-doors.
This is the important part:  when the doors have closed, go into “sneak” mode and creep over to the dark area under the stairs to your left.  Ready your fiber wire and wait.  The guard will soon come back out through the double doors.  Strangle him, drag him under the stairs, and take his clothes.  Be quick about it, the maid will be back all too soon.  Then, move through the door to the right of the entrance and make your way to the kitchen.
In the kitchen, retrieve your guns from the box the delivery boy brought in.  Go up the kitchen stairs, being sure to avoid people, and make your way to the hallway outside the Don’s room.  Walk just to where the corridor turns to the right.  Ready your 9MM silencer pistol before you turn the corner.  At the corner, sidestep and nail the guard in the head with one shot.
Watch your map for a good time to enter the Don’s room.  Again, you should be able to nail him in the head with one shot from your silenced pistol.  Take the key and split.  Again, make sure you dodge people.  Make your way down the kitchen stairs and down into the dungeon.  Check out the cell.
To leave, I took the door near the garage.  The guard standing near the door wasn’t moving, but no one was near him and I got impatient, so I once again shot him once in the head with the silenced 9MM.  I then walked out the door and on my merry way.  Mission accomplished, three shots fired, no alarms raised.
This method didn’t earn the “Silent Assassin” rating, but it got the job done.  Besides, getting “Silent Assassin” on the next two levels is easy enough.  On one, I nailed two targets with one sniper shot.  Booya!