Quote Originally Posted by TinCow
First campaign:
I grabbed Zagreb and Durazzo almost instantly, with Florence and Rhodes following shortly. I made alliances with everyone I could and was raking in money. Unfortunately, my military forces were relatively minimal, with the only sizable force capable of taking the field located near Venice. Hungary and Byzantium allied and both declared war and besieged Zagreb and Durazzo respectively on the same turn. Both stacks were massive and I had no chance of defeating the assaults. I sent my large army from Venice to try and help Zagreb. Both cities fell the next turn, before my relief army could arrive. With the main force diverted to the east, Milan (my allies) declared war and besieged Florence. Hungary also moved on Ragusa. I turned the army back around made for Florence, but it too fell before I could get down to help it. HRE then declared war, defeated my main stack and took Venice. I had managed to assemble a large force on Crete and could have attempted to invade Greece and survive that way, but instead I declared defeat and started over.
I lost my first campaign in very similar circumstances. To avoid a repeat, I did two things; Firstly, I reigned in my conquest of Rhodes till later in the game, concentrating on consolidating the Adriatic first (the Hungarians WILL attack if you look weak). I also converted Duosso into a castle to stop it revolting and to free up units.

Second, I took out Milan immediately. It took about 10 turns and allowed me to focus on Bologna, Florence and Sicily without having to worry about Milanese armies. From that point, winning the game was a foregone conclusion.