Since it came up as an issue of why ZK is weak and difficult to present to new players, I've decided to write in more detail about how/why the missions and campaign are terrible.
Yes, terrible, and "free" is only an excuse that does not fix the problem. So here goes.
Tutorials:
If you play any other RTS the tutorials are usually extremely simple.
Here's some stuff to look at. Here's how you control the camera.
Here's some units, here's how you select them. Ok now here's how you move them. Here's how you fight move, line move, etc. K Tutorial over, on to the next.
In ZK from the very first tutorial things are way overcomplicated. Moving a commander through a battlefield is not only onerous and stressful, but it fails to teach basic things like camera controls, fight move, line move, queueing commands and so on. In the second mission you get random unit counters thrown at you while you're trying to learn just to build mexes and solars and maybe produce your own damn glaives from the fac you took forever building since you got no plop, but no it won't even leave you alone enough to manage that. And then you get to fight a DDM!
FFS. Even for someone who has played RTS games before it's highly abrasive. Missions need to be simple and they should not try to teach more than one thing at a time.
And then even assuming they make it past that nonsense, once you hit the 1v1 CAI mission its like all your hopes and dreams get flushed down the toilet. Comparatively the previous mission (2v2) is a cakewalk and the most important things you need to win (basic eco and raiding) aren't taught in any of the previous missions. Not to mention you go from having almost nothing available to suddenly having every fac, item of porc and so on become available at once. For a newb to figure out that titan duel is a veh map, that they should scorcher rush, or for them to know how to manage their eco efficiently, just isn't going to happen and the result is brutal. Also difficult missions like that really need their own category, like the "challenge missions" in SC2.
Campaign:
The current campaign is really weak for a lot of reasons.
First of all the story is entirely incoherent to the player. The player doesn't know what the planetwars are, or what the empire is, or what the promethean is, or anything really. Those things are just thrown at you like you're supposed to accept that it will eventually make sense (which it never does). The only part of the story that really makes any sense at all is the part where you're conquering the random bot clans and leading them against the pseudo-empire, and only because that's what you spend the majority of the campaign doing. Everything else is incoherent gibberish completely lacking in context, starting from the first mission.
The fact that the campaign takes place
after the planetwars is also a bad setting. The planetwars, the involved factions and the events surrounding them obviously are more interesting than what happens in some middle-of-nowhere bunghole a thousand years later. Particularly since the planetwars refer to an actual multiplayer campaign event type of thing it only makes sense that the campaign should introduce them.
Another issue is the complete lack of humanity anywhere in the script. Every mission Ada basically goes "Haha, some stuff happened but it was easy cause I'm a 1337 c0mm4nd3r from the empire! Serves you right, savages!". There's no real struggle in the story, no factions or people that you can identify with, no revelation of interesting events or history, nothing relatable at all.
In terms of gameplay the campaign missions tend to fall into one of two categories, either they're too easy or ridiculously difficult to the point of requiring some sort of gimmick to win. Also, while the campaign missions should be teaching players how to use each factory and how to counter things they're often set up such that units you really need are disabled, or such that the units you get are made useless.
For example in the mission on ravaged where you play shields, if you build thuglaw you can kill the null AI but if you do you die to the CAI which builds counters. If you build bandit/rogue you can win but you won't be able to kill the null AI because it has too much porc. You really need felon or racket, but both are disabled.
Another example is the jumpjets mission where the CAI is killing you with rogues and all you need is some puppies but they're disabled for no good reason. That mission was really an exhibition of bad design.
Another example is that tanks/veh mission where the CAI sits behind a wall of DDMs on hills. Not only is that mission ridiculously difficult but impalers are disabled when they're basically the only thing that could hit the DDMs. Because of that most of the units you can build are useless and you end up having to grind over it very slowly with wolverines and pillagers until you can punch a tiny hole in one side of the DDM wall to get to the CAI's fac. Eventually it enables silo but even getting a silo within range is ridiculously difficult and slower than just grinding over it.
Another example where the mission says one thing and does another is the king of the hill mission. The 'secret' to winning is to raid early on to give your allies the advantage, but the mission never tells you that. If you do what the mission says and just try to dump units/porc into the middle your allies die and you get run over and you're given no clues as to why.
I guess the major points are:
-Each mission should introduce something new. (and not just units but also concepts like eco, raiding, proper defense and how to use a given fac strategically)
-The units you get in each mission should all be useful and not be directly negated by the mission setup.
-If a mission produces situations that really demand a certain unit then that unit should not be disabled by default.
-The factory the player gets for a given mission should match the map well, and this should also be explained to the player so that they can learn what fac to build on what maps.
-Missions should gradually pit you more directly against the CAI, and eventually replace it with smarter AIs.
-Missions should try to avoid using contrived situations to provide difficulty. In other words the player's disadvantage should never be too severe or artificial.