I was playing 'Medieval 2' as a kid, now I'm 23 and decided to pick this game up again. I played vanilla as Poland on VH/VH and it was very fun. Until recently someone told me about Stainless Steel mod.
So I was playing Stainless Steel on VH/VH + 'Byg's Grim Reality IV' + 'Real Recruitment' + 'Limited Activities' + 'Longer Assimilation' and here's what I want to say to everyone who's like me and wants to check Stainless Steel mod out:
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1. Is 'Stainless Steel 6.4' worth installing?
In my opinion yes, very much so. The bigger map is amazing in actual gameplay and new units add much more depth and fun to the game. Vanilla units tend to feel like they are all the same across different factions. Stainless Steel units feel awesome and every faction brings a lot of different units.
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2. Should I play with 'Gracul AI', 'Lusted AI' or 'Savage AI'?
I suggest 'Savage AI'. From my experience it's only a little less aggressive or as aggressive as 'Gracul AI'. But it's probably more reasonable than 'Gracul AI' because 'Gracul AI' is just senseless everlasting hatred towards the player. Especially from your own troops as they tend to rebel all the time on 'Gracul AI' to a ridiculous degree. 'Lusted AI' is probably too easy.
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3. Should I install 'Byg's Grim Reality II' or 'Byg's Grim Reality IV'?
Don't install any of them. Uncheck both of them. In my opinion BGR is simply a bad mod.
The reason why BGR is IMO a bad mod is because while Stainless Steel makes the game harder (in a right way) an also very fun too. Meanwhile BGR will make the game a chore.
I play as Poland - this faction has weaker infantry and foot archers, and is constantly under Siege from all directions (Holy Roman Empire to the west, Denmark from the beyond the Baltic see, Lithuania and Kiev and Novgorod from the east and someone may join the fight from the south too). To defend yourself you will have to constantly recruit new units as fast as possible and move them around the map all the time.
So in practice what BGR mod will do is simply punish you heavily for everything. You have to pay heavy money to supply troops when they travel across borders, you will have to send your 3 best generals to sit like ducks in a few towns and recruits units (because only they those 3 best generals can recruit best units and if they leave that town, new units won't be waiting for them ready to be recruited - their "turns to next available unit" will reset).
So the beginning of a campaign is OK but as soon as you get your first/second war declaration you will constantly lack troops, lack money (you will also get into a heavy debt due to paying tons of money for supplies) and you won't appreciate that "deeper economics" anyways... Plus BGR will also make waiting for your next turn longer...
This mod should actually be renamed to "Byg's Pointless, Masochistic Reallity" - it would reflect that mod more accurately.
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3. Should I install 'Real Recruitment'?
I would suggest not to use it. From what I saw it's probably a chore similar to BGR mod. You won't be able to use good and fun units until the campaign is very late or it's too late for your faction to survive as you are already overwhelmed by enemy armies that have access to better units (at least better than Polish militia) and they can make a lot of them. Or at least much more and better of anything than you can do.
Someone even wrote on some forum that he wanted to play the Holy Roman Empire for the Gothic Knights... And in 500 turns campaign he got access to them somewhere on turn 450... and only in a few areas of HRE.
So it's simply not worth it. In 'Stainless Steel' the better units are already much more expensive and take a lot of time to recruit. No need to make them non-existing with 'Real Recruitment' enabled.
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4. Should I install 'Limited Access'?
This one can actually be fun. I played the game with it and it was pretty fun. You will have to move your generals from town to town so it forces you to plan your town development for future turns. Nonetheless, it gives a bit more difficulty for that reason so I suggest to check it out but can as well uncheck it if you want.
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5. Should I install 'Longer Assimilation'?
IMO yes. What it does is forcing you to stay with your army in a conquered town for a one turn longer in order to do establish your control over it. It's pretty fun in gameplay.
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6. Should I install 'Permanent Arrows'?
Depends on your computer. If you have a good PC, then you can go for it. I myself never really payed attention to tiny arrows in the ground.
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7. Should I install 'Permanent Watchtowers'?
IMO no because it's quite fun to chose strategic locations for the watchtowers yourself.
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8. What campaign and battle difficulty should I play on?
When it comes to Battle Difficulty - play on Very Hard. This is really fun and quite fair experience. Most battle will resolve around killing enemy General to win. Otherwise there is no chance for enemy army to route. It may seem hard to do but it's actually nicely done in practice (but keep in mind that enemy's General's Bodyguard are very hard to kill in Stainless Steel).
When it comes to Campaign Difficulty - it depends on what faction you play as. An isolated faction will be easier to play. The higher the difficulty the more armies the enemy gets out of thin air to attack you. Check it out yourself but I suggest starting at Very Hard, and then eventually lowering the difficulty if need be.
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So this would be my opinion after trying playing Stainless Steel as Poland (which is a difficult faction to play) with all possible difficulty. But the highest possible difficulty is simply not worth your time. Just do as I said above and the game should both very fun and satisfying.