i'm experienced... first make sure you upgraded to 1.2.
I actually find the game ridiculously easy, I play hard/hard, haven't lost a fight in my current campaign and have just defeated the turks and byzantine's. My current campaign is egypt, my first was england.
I really didn't play the game to my full capacity, so some of the advice will involve things I didn't actually do. I tended to find the diplomatic side of the game unfulfilling, so I avoided it for the most part. Venice's challenge is you're surrounded my other catholic nations. As a catholic you really should not engage in combat with other catholic's as that is what will bring down the pope's favour.. but the diplomatic side of the game is so buggy it's hard to give advice. For instance in my game I had an awful reputation despite never doing any diplomatic actions beyond trade rights, and having never once attacked another nation first. Since you're playing easy, then maybe the AI will be more forgiving...
I'll tell you what I'd probably do as venice. First thing you're focusing on is taking the rebel towns, you can search around with spies & diplomats, and you'll sometimes get assignments. Unless you see some real good trade goods, i wouldn't bother with merchants to start. Make sure that when you do go to take these towns, you're taking those adjacent to you. If you start wandering around on others territory you'll piss them off, and try not to pick fights. Produce like 3 diplomats and send them in opposite directions and go around to the factions and get trade rights. As I mentioned before, messing with other catholics is bad, so you'll probably wanna do 2 things. First is establish alliances with the other catholics, secondly put yourself in a position where you can fight the muslims.
You start with a town on an island, NE of there on the mainland there's some rebel towns. From there you can put yourself right beside the turks and engage in some ass whipping, beat them and go south to egypt, and then west to the moors. Anyways just some ideas, you may find fighting the other catholic nations more enjoyable.
Few other notes, I have way more towns then castles. I really only have a castle if I need it since it brings in less income. So for instance I'd say have one castle by my starting towns, all my islands are towns typically for the trade, and the computer sucks at naval invasions anyways. And then as I expand I would guesstimate I have one castle for every 4 towns typically. If I take over a castle from the enemy and I already have a castle right beside it or whatever, i'll typically convert the lesser castle.
Combat is rather simple most of the time, every so often you can be creative... but typically horses at flanks, archers at front... since you might go into muslim nations I suggest light calvary, and your own archers to negate there abundance of horse archers, especially when the mongols come...