Hiya, eefums, welcome to the board.

Now, on to your question:

The transition to MTW isn't going to be a huge change of style. Yes, you will now have to choose whether to have a city (money-focus) or a castle (military-focus) for a region. Yes, diplomacy will be more important. And yes, Religion is gonna be a big issue. But the mechanics are going to be the same for the most part. You will siege towns, you will still beat armies/

Religion wasn't present in RTW and in BI, it was there, but not on a grand scale. In MTW, religion is an import aspect. Catholic factions aren't gonna be great mates with Muslim factions unless they want to have the Pope breathing in their neck. And if the good old Pontiff has a bad day, he may even consider you a heretic and urge the other Christians to attack you!

Crusades are a bit like the senate missions in RTW. You are going to attack a town in far away land, you will get some bonus units (powerful crusading units like the Templars) for the job and you will gain prestige with Pope, which you can use to do more crusades (on targets of your choosing) or use it for attacking other catholic factions.

The Pope is the Senate: he gives you missions and can be a pain in the backside when you ignore him. In Rome, you could wipe out the senate (it was a requirement to win the game) but in MTW, eliminating the Pope won't work (unless something changed).

Regarding unit tactics, well, it is the Medieval period. You will some bad-ass heavy cavalry, but you will also see some bad-ass counters to those horsemen. Every unit class will murder one unit class, but be run over by another unit class. For example, swordsman are fabulous against spearman and pikeman. Cavalry on the other hand can ride down swordsman with relative ease.

These are some aspects that are a bit differen from RTW. If you have any further questions, please post them and we'll do our best to answer them.