Overview

Venice are in an interesting position. At the start they border Milan, HRE and Hungary, and soon will border Byzantium and Sicily as well. It has good trade and a good mix of units, especially some useful militia units to protect its cities. It's unique infantry units(Venetian Heavy Infatry, Venetian Archers) do not take long to get, and are good quality troops.

Starting Position

As mentioned above, your borders are nearly surrounded by other factions at the beginning. There are 3 rebel provinces near by that you should try and get as soon as you can. Zagreb or Durazzo you wil be assinged to take by the Council of Nobles, and i go for both of them. Also try and get Florence if you can. Also try and build up good relations with the other factions in Italy, as well as Hungary as you do not want them to backstab you if you play Venice like i do.

The Pope

Do everything you can to make yourself the best loved faction fo the Pope. This greatly helps you out with wars against other catholic factions. Milan are likely to back stab you at some point, but maintain good garrisons in you settlements and let the ai attack them. This wa you beat up their army, they are the agressors, and will 9/10 times accept a ceasfire the next turn.

Byzantium

Once i've got myself quite secure in Italy, i go after the Byzantines. They field quite archer heavy armies, which can be a pain as Venice, but if you bring armoured sergeants, mailed kngihts, and your own archers along their armies should be no trouble. I plan my war against Byzantium in 2 parts.

1. Take Thessalonica, Corinth and Constantionple. Then build up my forces before
2. Taking the rest of their settlements, as well as Rhodes.

Once Byzantium is destroyed you should have enough provinces to be raking in the cash.

The Holy Land

What i like to do next, is use Constantionple, Cyprus and Rhodes as launch pads for Crusades to the Holy Land. This further extends your empire into provinces which can bring in a lot of cash, and also provides you with large armies for when the Mongols come.

Italy

All this time you will more than likely have been attacked in Italy. At first just beat them when they attack your cities then make peace. Later on try and capture a province from the factions that attack you every now and then. Then rebuild your standing with the Pope. This way you can slowly conquer Italy, whilst not getting ex-communicated.

The Mongols and Timurids

These 2 can be either a huge pain, or do nothing. Personally i plan based on the huge pain thing. Let them attack your cities, and beat them when they assault them. In open battles their horse archers and cavalry will beat you, even if you have got to Chivalric Knights and Menat arms by this point. Wear them down slowly using your fortresses.

End Game

You should already have quite a few of the 45 provinces needed for victory, and now you can do several things, or a combination of them.

1. Conquer the rest of the middle east and Egypt
2. The New World
3. Beat up some other Catholic factions to get the required provinces.

I hope this is helpful, even if it is a bit short.