The Youtube content creator poqreslux once said, while reviewing the 3.3 version of this mod,: "the more you make Medieval II not resemble itself, the more it tends to poop itself". Truer words have never been said. In essence, the Medieval II's engine is the primary culprit in the instability of mods, just by the virtue of things being changed from the vanilla game. In addition to that, Medieval II runs on an old engine and is not optimized for modern systems or hardware. So, for the most part, stability issues are outside of the modder's hands and to make mods as stable as they can possibly be is primarily dependent upon the person playing the mod and their individual system. So, with that in mind, here are a few things that you can do to ensure that you are not adding the instability of the mod and are instead minimizing instability issues on your end.
1) Do NOT Install Medieval 2 Total War in the standard library of C:\Program Files\Steam... In fact, do not install in either Program Files or Program Files x86. The guys at Europa Barbarorum 2 made a great guide (see: 1. M2TW Installation Instructions) on how to install and/or move your Medieval II installation outside of Program Files, whether using Steam or not. Having your Medieval II installation in Program Files has been known to wreak havoc on mods, causing a variety of issues, and as such I cannot help you if you experience bugs or crashes but fail to take this important step.
2) Short your directory names. There is "old code" M2TW bug which can cause CTDs if the complete path to a file exceeds 128 characters in length. The team has made extensive efforts to reduce the length of file names, but players can still cause this bug by using a "too long" game installation path. Accordingly, your installation folder directory names should be as short as possible. Keep this in mind in conjunction with #1 above.
3) Install the 4GB Patch. I have said this a thousand times, and it is in the mod description, but this is a necessity to play this mod. Medieval II's engine cannot normally use more than 2GB of resources - the 4GB Patch obviously doubles that limit. The easiest way is to Google "4gb patch" and the first result should be from NTCore. When you run the 4gb_patch.exe, apply it to medieval2.exe
4) Be aware and realistic of your system's operating capacities. A large portion of crashes and errors are due to hardware issue - your system can't handle what it is being asked to handle by the mod - which comes in a variety of areas. RAM, available drive memory, video card, processor, operating system, BIOS... there are so many things involving one's system that have an impact on gaming in general. Considering the age of Medieval 2's engine and that it is absolutely not optimized for modern systems, and that the more you make Medieval 2 not look like Medieval 2 the more unstable it becomes, to have large-scale total overhaul mods run well (with limited crashing) you have to have a relatively good, modern rig.
5) Run in windowed mode. This does not apply to everyone, but for some people the mod appears to be a lot less stable in full-screen than running it windowed - particularly if you alt-tab out of the game. If you get fairly regular crashes, try running the mod windowed and it could probably work better for you!
6) Turn off auto-save and make regular manual saves. There is a known M2TW bug which results in corrupted "autosave" files and deactivation of parts of the mod's scripts. If you are experiencing an issue with autosaves being corrupted, or malfunctioning aging or events, the solution is to turn off the autosave function and remember to manually save your games frequently. However once a save is corrupted by loading an autosave, the script issues and corruption cannot be repaired on that playthrough - you'll need to start a new campaign.
7) Avoid loading and saving in specific circumstances. If you make a mistake in your campaign, and you are tempted to use the latest autosave, I would highly suggest not doing so. Instead you should use your latest manual save (preferably quitting to the main menu first) or just continue on with your mistake in your campaign. Loading autosaves in the middle of a campaign (or possibly using them at all) can cause you many script and corruption issues and make aspects of the mod not work properly - in that same vein, loading any saves (even manual) in the middle of the campaign could cause issues, so best to quit to the main menu and then load a save (just to be safe). Another thing to avoid is not clearing out all of the event pop-ups (bottom left of screen), especially for missions, before making a save - attempting to load a save that has event pop-ups that have not been cleared can also cause issues with scripts related to those event pop-ups (such as missions). And lastly, do not save during the “pre-battle” screen and then try to load that save (to replay a battle) as it will more than likely crash the game when attempting to load either the save or load into the battle.
8) Alt + Tab. For some reason, sometimes mods will 'hang up' when loading. This can happen either when loading a savegame or when an AI faction is processing their turn. If in either of those situations, and it appears that nothing is actually loading (ie: you don't see the loading bar when trying to load a savegame), try to press Alt + Tab out of the game, then open it again. Whatever wasn't loading should start loading once you return to the game. I'm not sure why the game won't begin loading in specific situations, or why using Alt + Tab fixes it, but it does.
And to get this to post on the page, I apparently have to include two images... so forgive me for including two that have already been published here.
Nice guide!