In pretty much any RTS campaign, you are both outnumbered and outstupided by the AI. You are expected to capitalize on AI's stupidity.
Some campaigns are harder than others. Original TSK shouldn't be too bad. Afaik, there should be no attacks you cannot stop with starting troops in 1st hour. Getting a town that can feed itself and produce soldiers in that time should be quite easy, if you know what you should do.
If you aren't sure what to do, assuming playing remake:
Don't deliver stones, coal and corn to storehouses. Stop storing things you use in a production chain - the only things that always should be stored are food, pigs and skins (if a swine farm has 5 of either, it stops producing). Store timber until your wood weapon production is up/you have built all you want to build.
Make 4 or 5 quarries, at least 2 fully operating sawmills (4+ woodcutters), 20 laborers, get food and gold production up before running out.
Farms and vineyards need some time before they start producing. Build woodcutters close to trees, otherwise they too take a while before they start producing (TSK mission 14 is bad in that regard - first enemy attacks are rather bad and there are no trees around - make many woodcutters asap, you can't wait too long for wood)
Make related buildings close to each other - stables and swine farms close to farms etc. Minimize carrying distances - high input+low output buildings close to their input and vice versa. Don't worry too much about optimal building placement and laborers walking around - optimizing every step isn't needed in original campaigns, just try to get laborers working most of the time and building chains reasonably close. Don't store wares from production chains far from storehouses (except pigs and skins) - bakeries and butcheries should be close to inns AND storehouses.
Make bread - in singleplayer, you need to feed army and additional serfs for feeding army. 2 (or 3 for a big city) bakeries should be enough.
If you can get food, gold and weapon production up at around 1 hour mark, it should be good enough.
If you notice a building blocking something or causing a major traffic jam, demolish it and build it elsewhere.
You should be able to make more soldiers than you lose to attacks and grow your army - make sure your food production and serf population can keep up. In singleplayer, you want to keep (and feed) your soldiers. Don't bother with attacking until you have a sufficiently big army. In fights, try to outnumber your enemy - this is mostly about soldiers fighting at once. If enemy soldiers are all around the place and yours are concentrated in one place, you can win with smaller total numbers.
This is far from perfect, but should be good enough for start. If you can do that, you should be able to complete at least TSK building missions by actually building a town and making an army. If you can't get any reasonable production up in 60 minutes, you won't stand a chance in multiplayer too (like 60+ soldiers and a decent production at minute 90; at minute 60 for "good" players).
Also keep in mind that some maps (and campaigns) are meant to be challenging for experienced players. In TSK campaign, there are 2 troublesome missions I remember: 8 and 14. In 8, you need to survive the 1st attack - build school and watchtowers (nothing in between). 14 has rather early (and strong) attacks and you don't have quite enough wood for a comfortable start.
When it comes to multiplayer, this sums it pretty well:
https://www.nerfnow.com/comic/513
It will NOT be easier than campaigns and you will have to know how to optimally build and fight to succeed.
Btw, there are plans to have difficulty levels in future remake versions, but last time I saw them they were very wip (a while ago); there's nothing in stable r6720 or in original TSK and TPR.
Also, what cmowla does is pretty much pushing AI abuse to the limits, and it is a complete overkill for any remotely reasonable map/mission. This is beyond merely playing, this is the art of absolute optimization by any means available.