Just started playing the game and picked 100% (Challenging) difficulty for the first mission. At day 22, I was hit simultaneously by two huge swarms. You can guess what happened.

I figured I was in over my head and tried again at 50%. This time I get a single swarm, maybe one-tenth the size of the first attempt's swarms combined. Ie a huge difference.

Can anyone tell me if this working as intended or is something borked in my savefiles, setup or game installation (1.0.14) ?

In my own experience (the same game version) swarm missions can and will overwhelm you on challenging. The beginning maps are perfectly doable if one clears the area of zeds proactively. First the immediate area around your HQ, then nearby and only then after training a couple of defensive units for reaction to strays, send groups of units to clear the map since when the swarm attacks even if you resist it there will always be more zeds that get attracted to the noise.

Later on you might get frustrated if you don't pick the right techs in the research panel (in the campaign) since the difficulty builds up on the later maps in the sense of different types of zeds. Personally i found the economic objectives (build up x number of population) in addition to resisting the swarms more annoying.

I know -- that is just the game working as intended.

For comparison, I just watched a let's-play of the first level on Youtube, also at 100% difficulty. The player got a somewhat-large swarm that was nothing like the two huge hordes I got on my first try.

Anyway, I wiped the My Games\TAB folder and started a new campaign. Everything seems to work okay now, so hopefully this was a one-time glitch.

UPD:1 GOG version is broken. Last mission CTD almost at the end.

UPD2: Look at this web tool - keep in mind that you can get only 10k point maximum for every missions before last one (and 9 hero perks before 10 hero mission)
http://melanite.net/tab/
Also keep in mind, that game probably have some bugs\missinfo about some techs. And this tool is kinda outdated.

UPD3: I did recheck that calculator... After you check only tech itselfs (new buildings\units) - you spent almost all points. There is no points for upgrades (building or units) additional to ones that required for new technlogy. So, last points spent to economy - train. +1 (of 2 possible) gold per zombie kills and +30% cost return for sellings buildings. No additional units at start, no +any% armor for soldiers.
Something like that (80 points last, but there is nothing to spent them)
http://melanite.net/tab/?order=050c160d0207040a22121a1b21242931370e151d2a414e5b25320f1720282f393f4b26272d332e2b3c3036353a3d4249483e40464d50541e111913434a472c

I want 4-6 more upgrades: two add/ units, +20% armor, +1(of two) shock twoer range, and +1 second gold per kill. Less useful is extra ballist at start.
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Since very frst mission is lmost like Tutorial, probably better play it on lowest 25% difficulty. Just for introducing. 500%\800% (or wich ones are maximum? i forgot that) difficulties added some later after 1.0 campaign release along with mission replay function. This is for replay previus missions with more tech's\hero leveling upgrades. If you not want cheats like me (see below), play firsts missions initially maximum on 300%.

Randomised player stress-tests (must be in skirmish survival only?). Scripted pre-swarm stress-test. When last swarm incoming - every remaining zombies start moving to player HQ.

If some previuos version played, My Games\TAB must be entirely deleted - and start new game. But first, if you use GOG Galaxy or Steam version (or else online) - check game files. At least twice. For Steam v/ Steam must say that all files are correct - without redownloading (not "1 files to be redownlaoded").

There is some cheat trainers. Or CheatEngine or ArtMoney tables. Or just use ArtMoney to find in memory your Research Points after 1-2 missions. By using ArtMoney you can get all 14.250 points (insted of 10k). You can switch difficlty to 800% and play with every points further. Or try to balance researchs. Just keep most pints in case if you think you mess with tech tree.

You must research central line in three, and "train" line. Also find where +40 soldiers armor, increasing buildings HP (green bar not matter, you MUST increase yellow by this upgrade), upgrade Tower building in Tower Defense Hordes missions.
Even with that - you must research-upgrade wood walls\etc and later in game - stone walls\etc. This is one of the fundamental problems in the game: you must research earlier tech's for firsts missins - but other research for later missions. And game not allow to reset all points. This is why i choose to cheat in campaign. But trying balance: only 800% for every missions (hero\hordes too) snd trying to balance firsts tech research.

Very first upgrades - Wood Houses. You cant win missions somewhere after begining only with tents.

In campaign you start with 4 archers and 1 soldier (after you research them - must have). You can add one more archer and one sniper. Total 7 units at start. very useful. There is also 80 points +1 more soldier. But this tech not onclude any real tech's upgrades (others additional units have additional upgrade or lead to next tech upgrade). And 8 units at start = only 2 workers at start. Cause HQ have only 10 citizens-workers. Better keep 7 units.

Soldiers - main army unit. Even without armor upgrades (20+40+5 after every tech upgrades +10\15% for fire\poison resistance). And careful with Lucifers flamethrowers. For oil tech units main is Titans (good damage, good speed (not like Mutant)) and Thanatos (good splash damage).

Mutant - very cool unit. Mybe even not by his splash damage - but fr his speed. Two of them can later in the game save the day if you miss something in defense on one of the flangs. There is also enemy Mutant - they stronger. :(

Upgrade horde-missions Tower - must have. One of the latest mission - with Gigants+Mutants. Mutants if fastest. They need only one hit for not-upgraded Tower.
Also, i think every of 6 special building are must have too. Well, maybe not Bunker (use it only as addition to bank)...