How do you open the doors of the doges palace on assassins creed 2

You cannot enter the Basilica San Marco (Doge's Palace) by any means except by Leonardo Da Vinci's glider. At least until you learn the new method of jumping to a higher ledge while climbing, but the first attempt will be by air. (a cool level, I might add. Fortunately, you get to fly it as many tiomes as you like in Forli once the "Special Memory" is unlocked.)