I found the Papal states to be of great use in this campaign (as with any catholic faction). When playing catholics I tend to get an alliance and military access both ways with the Papal states ASAP. It's fairly easy to keep in the Pope's good books after this, by building churches and priests whenever possible, which is good for morale as well, and self-reinforcing, as it improves your chances to control the Papal elections (so far in every game I've played, every Pope after the first one has been from my faction) thus keeping Popes favorable to you in power. Step two of makes you even more popular with the Pope.

When I find myself in a difficult region, with multiple factions bordering me, leading to strained relations and possible military engagements I'd rather avoid, I use the Papal states to create a buffer state. The Pope has always been a great neighbour, with the aforementioned treaties I've never had to worry about Papal armies in the slightest, indeed they've helped me out more than once. This also makes living with the other factions a lot easier, as they're happy to ally and trade with you as long as there are no border clashes, and they're loath to attack the Papal states regions, or to send troops across them. And if they're not catholic, or are sufficiently angry at the Pope to attack, then you have an excuse to launch a crusade against them, restoring the buffer state, and knocking them down a few pegs in the process. If the buffer zone becomes more of a hinderance than a shield, just send troops through to attack the regions on the other side of it with relative impunity. I have Papal states regions dotted throughout my empire, with no problems at all. The only downside is having to follow the basic Papal mandates, so you may not always be able to attack the catholic regions you'd like to. Still, you can generally manoeuvre other rulers into getting themselves excommunicated, take their provinces through diplomacy, or wait for them to otherwise make a mistake. Failing this, spies and a good army mean you can take a city faster than you can be excommunicated, as many people have already pointed out on the forums.

On to Venice.

With Venice, pretexts to fight other catholic rulers aren't much of an issue outside of the Italian peninsula. I decided to focus on the Mediterranean, and develop as a naval and trade power. My main early goal was to expand along the coast to Constantinople, thus ensuring that my main enemies were orthodox or rebels. I also grabbed Florence, sending an army through the Empire territories to do so, in order to keep a favorable power balance in Italy.
To the north of Greece, Zagreb and Sofia can be gifted to the Papal states in order to create a massive buffer zone protecting you from interference by Hungary and the Empire in all of the region. I held onto Zagreb, as it's my only castle in the area, not wanting to sacrifice any of the coastal provinces for this task. This caused a lot of headaches with both the Empire and Hungary, eventually leading to a short vicious war which could probably have been avoided, and has benefitted me very little, as I still haven't had much use from the Zagreb troops. I engaged in extensive diplomacy in the opening turns in Italy, gaining alliances with all the local factions bar the Empire, although only the Papal states had a full military access agreement with me. A possible early move would be to buy Bologna (from memory, may actually be a different region, the region directly south of Venice) from the Empire before it is built up too much, as this would save a lot of friction. I didn't manage this either, another cause of the later war. Still, intensive diplomacy managed to keep northern Italy in an uneasy peace until I had conquered Constantinople, reduced the Byzantine empire to a handful of far flung colonies and gifted Nicaea to the Pope, giving me a buffer from the Turks.
Around this time, Milan betrayed our alliance, and started a pointless blockade on Florence. Their land armies didn't press the advantage, and I was able to snatch both Genoa and Milan (with the help of a good spy) from them in one turn, placing them firmly on the back foot. I consolidated this advantage by quickly pushing forward to Marseille, which became my western Papal states buffer, keeping Milan, France and Spain out of my affairs. Now separated from me by this new buffer state, the Milanese were quick to agree to a ceasefire, which lasted until I had a force in place to take over their island holdings, reducing them to a land power. This damaged my standing a fair bit with the other factions, and noone trusts me any more, but since I'm ranked first across the board, it's not much of an issue.
With east and west secured for the time being, I turned my attention north. Abysmal relations with the Empire (which now had both Milan and Hungary as vassals, thus taking up 100% of my northern border) had deteriorated into open war after the Milan crisis, a few clumsy bribe attempts on Bologna and finally, a failed assassination attempt on an Imperial family member (who was sitting a few miles from Milan with a decent sized army). I took advantage of the opening turns to smash Bologna with three armies from Florence, Genoa and Venice, and then sat back to defend my northern border. Not long after, the Emperor had been excommunicated, and I was able to call a crusade on a city deep in the middle of Imperial territory. In this short but nasty war the Empire lost around three-four stacks of troops, and the regions of Innsbruck, Vienna, Budapest (the last remaining Hungarian region), and I lost two stacks of my own. When the dust settled, however, I had a solid Papal border consisting of Marseille-Innsbruck-Vienna-Budapest-Sofia cutting me off from the whole of catholic Europe. The two Imperial territories I took beyond this buffer I gifted to Scotland and Spain, improving relations with them dramatically, as well as making northern Europe even more chaotic.
Since that war, the only catholic opponent I have had has been Sicily, who I have methodically ousted from Italy, Sicily and northern Africa as I attempt to dominate the mediterranean. Progress is slow, as the Pope doesn't approve, but city by city the Sicilians are being wiped out, and Papal relations are good due to all the huge cathedrals being built with the mediterranean riches.
Now my attention has been drawn back to the east, with the turks warring on the Pope in Nicaea, I went to his rescue, and met the Mongols coming the other way. Currently I am bogged down in a massive war of attrition in Turkey as I wear down the Mongols and their damned full silver chevron starting armies. One full stack of my troops can just about defeat one of theirs in the open (medium difficulty), which seems a good deal, as mine are raw recruits, and therefore limitless, and theirs are elites, and therefore limited. So far this is the only serious military opposition I've faced in the campaign, and I'm enjoying it, or at least I was up until, after a long and arduous siege battle where I managed to fight off a full stack of Mongols with a half stack of mercenaries and cavalry militia, the game promptly crashed as they tried to withdraw through a mountain :/

Future targets are to gain the upper hand against the Mongols, and spread along the eastern and southern Mediterranean. After that Russia is a good source of non-catholic provinces, or I hope to opportunistically pick Europe apart with crusades as various faction leaders fall out with the Pope, in order to complete my region quota. Or perhaps a few ships from Venice can set sail to America, once it becomes available.

Militarily I haven't got much advice to give. I'm still working out the ins-and-outs of the Venetian units, which are quite specialised. Pike militia have performed well, holding Mongol full silver chevron units far longer than my other units (my cavalry invariably vanishes under a pile of light horse, and dies to a man). Musketeers have shown promise in the first battle I fought with them. The army was decimated, but the muskets caused some serious damage before it happened, and I look forward to seeing them defending castle walls. Venetian heavy infantry give a good account of themselves, and work well with the pike militia, although I preferred dismounted knights for the flanks, due to their morale bonuses. The heavy infantry performed much better against cavalry though, the knights tend to get massacred if caught out by cavalry units. A half hexagon in a wooded area, with pikes on the front, supported by heavy infantry, knights and a carroccio, and with decent ranged and cavalry support (so far my default army is 5 pike, 2 heavy, 2 knights, 1 carroccio, 5 ranged, 5 cavalry) performs pretty well against the mongols. I tend to lose a lot of the ranged support and cavalry, the first because I prefer to sacrifice them rather than risk the integrity of the pike wall, and the second because I hit any weak spot I can see with the cavalry until the enemy routs or the cavalry are all dead (and since the mongols are so damned tough, even archers can absorb a charge, and give back a fair bit of punishment, so the cavalry tends to get steadily chewed up). Still, at the end of the battle, the mongols have taken heavier and more valuable losses. On hard/very hard this will probably not be possible though, as it's a pretty close battle on medium.

-Lobo

Congratulations on maintaining such a great forum by the way, the Org has been my first stop for TW information since Rome, and I wish I'd known about it when I had Shogun. I've never got round to posting before, but I figured it was about time I gave something back ;)