As @Jank has said, there is no end to what determined Magi can accomplish in a Crusade. Are your PC's going to participate in the 5th Crusade? Wikipedia is not a bad place to start. If you're having a hard time finding the right books that answer the right questions, PM me and I'll see if I can't help.
If you have Players who love history - this is great, have them do some legwork for you and point you to summaries or send you key write ups of castles etc.
What your PCs can accomplish depends on:
(1) The Levant Tribunal Peripheral Code: (Note, there is a 4e book that has a "peace" but is not Canon. You do not need to use this, you are the SG after all!) Recall the Code of hermes states along the lines of "I will not interfere with Mundanes lest I bring ruin to my sodales."
So the key thing about the local interpretation is what "ruin" is. Remember in the Code of Hermes, in order to file a claim, SOMEONE has to claim damages! (That person can absolutely be Quaesitor who suspects something!)
(2) The level of enforcement and observations from Quaesitors watching the Crusade Seiges et al happen. If they can see the code being broken in real time, your players are in trouble.
(3) How obvious or subtle the help is. The less obvious the harder a time anyone (Sahirs of Solomon) or members of the Order will have to catch the PC's.
The help can be almost anything. Enchant soldier's armor so that it grants extra soak, reducing losses of soldiers. Subtly compromise enemy gates or walls with PeHe or PeTe spells.
Also, naturally, I have to plug my own Actual Play Campaign of the 1st Crusade that I'm happy for people to mine for material. Ars Magica 5th Edition: 1st Crusade - Actual Play Campaign
Long story short, the PC relied on using summoned spirits to accomplish a fair amount of shenanigans early on with subtle spells. Later, bigger spells relied on no witnesses surviving, and then the lone Quaesitor died... Giving the remaining Magi a much freer hand on what they did. Eventually, through legal loopholes, the PC had the Sahirs declared enemies of the Order and to be killed on sight. (Though this has yet to be confirmed by a Regional or Grand Tribunal.)
I also recommend Michael Fulton's Seige Warfare during the Crusades.
One tip I might add is using Mythic Game Master's GM emulator a try. Basically, you assign a narrative probability to an event i.e. "Not Likely" then roll a d100 to get a "Hard Yes" "Yes" "No" or "Hard No". So you can say "Are the quaesitors paying attention to this crusade?" Well, "Very Likely" is about right, so you make a roll on that. I then use this and make in game justifications for the roll.