This Assassin's Tomb is located in Romagna/Forli on the SW side of the City. The door is located on the outer side wall of the city near two guards protecting a chest. get on top of the wall and look down, you'll see a platform with the glowing Tomb door surrounded by water.
Make your way up, hit the lever and quickly run under the gate. Jump into the water and swim under the closed gate. Climb up the wall doing back jumps.
Kill the guards and work your way up. Push the 2nd and 3rd levers to open the gates and quickly jump to the other side. You'll see a series of shields, climb your way across the any shields you see.
Pull the final lever and jump your way toward the gate before it closes and you've made it into the chamber.