How not to design game mechanics: Lessons from CK2 combat system

I'm in the tiny minority of gamers who enjoys Paradox games. I often enjoy quite a lot of complexity in games I play - some of them like Magic: The Gathering are among the most complex games ever created (Magic's Comprehensive Rules are about 200 pages, and then add to it over 13,000 cards), and I just love discovering unusual interactions between disconnected parts of the ruleset.

Complexity in games can be not only fine, but amazing. On the other hand quite often it backfires horribly, and one really good example of such misuse of complexity is Crusader Kings 2's combat system.

How does it work

Here's a quick summary of just major CK2 combat elements by coanda. It doesn't really cover half of it like sieges, morale, army recruitment and upkeep, traits you can get in battle, chance of death during battle and so on.

Even that limited part of combat system looks really interesting at first - different unit types, multiple combat phases, interactions of different tactics, commander influence with separate commanders on each flank. Well, sure, there's the usual complaint that none of that is explained anywhere, but that's just the way Paradox games are. That's not the real problem.

The real problem is that nothing about the combat system translates to meaningful player actions.

Wait, what?

Different unit types are good against different opponents? Too bad, your demesne army is just a random mix of all unit types. Your vassals' levies? You have even less control over these mixes. Mercenaries? Almost the same story. You have a lot more control over your retinue, but then retinues are just overpowered without needing any special tactics and if a mod nerfs them down to reasonable size (like just about all of them do) they'll no longer affect your army composition significantly.

Different commanders having modifiers for unit types? Too bad again, all armies are just a random mix of everything, so it doesn't matter who you assign to lead whom.

Using terrain to give your mix of army advantage over opponent? (different unit types have different bonuses depending on terrain) The bonuses are pretty low and armies on both sides will be fairly similar mixes so it just turns into general terrain bonus equal to about their average.

And even if by some change you have perfect commander for your mix of troops - it still won't matter all that much, since he's only 1 of 3 commanders.

Basically everything a player can do other than:

• having bigger army
• appointing commanders with higher stats
• defending on difficult terrain

is nearly completely worthless because hundreds of positive and negative bonuses that different parts of player's and opponent's armies get for these fancy things end averaged up to approximately zero.

The result of this highly complex system is that it leaves all players feeling that "bigger army wins" and that's they need to know.

Simple grokable systems are the answer

Meanwhile EU3 had far simpler combat system - one commander, three unit types (infantry, cavalry, and artillery - all with tech-dependent fire, shock, and morale stats), everything expressed as straightforwardly additive numbers. 

And even though EU3's system was so much simpler - it led to many interesting interactions and it wasn't outside realm of possibility to defeat armies many times larger than your own by taking advantage of the system. (with navies even more spectacular victories were achievable, but AI stupidity also had a lot to do with it)

CK2's complexity doesn't lead to wonderful scenarios like "I'm going to bait French knights harrassing them with my light cavalry, then my spearmen will ambush them in hard terrain and finish them off easily" (the kind of stuff that's so common in all Total War games) - it leads to everything getting so mixed up that it's provides far fewer actionable options than EU3.

For contrast take a look at arumba's Clear Combat mod - it basically rips out the entire tactics system out of CK2 and replaces it with a very simple system based on just commander's martial ability. Does that reduce player's ability to do interesting things? Hardly! Unless you're playing with Mongol event troops (85%-cavalry army, and most of that horse archers) or some other event troops, you can basically do all the things you did before and they'll have pretty much the same effect, except now it's all much simpler and better presented.

Complexity is not a value in itself

Complexity in games is valuable if there are ways to interact with it - if it's just an excuse to roll a thousand dice behind player's back and average the results then its existence is completely pointless.

There are so many ways to turn CK2 combat system into something that players can interact with, and much simpler at the same time. Here are just the first few ideas that come to mind:

• Group troops in armies by type, not by levy origin. Then you could rearrange them on flanks the way you want and assign appropriate commanders to each unit type. (possible impossible to mod)
• At least have mercenaries of just one or two unit types, with big culture specific bonuses (like retinues) so you can use them this way. (that should be very easy to mod)
• Have very large terrain modifiers to different terrain types, so with even slightly more cavalry-heavy army you'd really want to stick to plains, while your slightly more infantry-heavy opponent would really want to stick to heavy terrain. (not too hard to mod, but AI will probably have no clue)
• Give different unit types very different uses, for example cavalry can't siege (like in EU3), only light units can loot. (probably very hard to mod)
• Have primary commander much larger impact over the entire battle, instead of just 1/3 of it. (I'm not sure how that works exactly)
• Have commanders or terrain or something else controllable strongly determine length or importance of battle phases. For example in forest you have no skirmish, just instant melee, on plains skirmish is twice as long, or something like that. And I don't mean insignificant 10% bonus, but something that would make a big difference. (not sure how moddable that would be) 
• Have an option to just harass enemy then flee. If your army is mostly light cavalry and horse archers, and their is mostly melee infantry that should let you bleed them a little at little cost then flee without actually losing the battle (and war score). Of course there should be a chance of this failing miserably if your commander is bad. (this looks doable with some heavy modding within existing tactics system)
• Just throw the complexity away like ClearCombat and put it in some other part of the game where it can be interacted with.