I agree. The singleplayer mode is frustrating and totally unbalanced. I quit after the 3rd mission. I wonder what the game designer were thinking ?? It's not fun to buy a game and leave it so early because is not fun but a pain in the ... :(

Who have you played against in skirmishes? Go against the Rat and the Sultan, they are the weakest enemies in the game. Against them, you can get a good grab of the game mechanics.
Hope this helps.
P.S. By "missions", do you mean the historical campaign or the skirmish trail?

Is the single player in Stronghold 1 similarly unbalanced? I'm only a few missions into the military campaign.

It's hard, but not unbalanced. You really need to learn the strengths and weaknesses of the different troop types and utilize them properly to win.
Go look up some playthroughs of it on YouTube if you like, they can give you some good hints as to what the best course of action is in general.
Most missions after the first few ones will probably require multiple attempts to beat, that's just the way the game is constructed.

It takes time but the AI is not too hard to beat - if you can get up a good defensive line the ai finds it very hard to beat through untill you get ot later stages where there are several agasint you at once.
Get the archers into the stronghold at the start and if you get attacked pull your troops back to here first - that way they get a defensive bonus. Don't bother trying to protect the outer areas unless the opponent is targeting them = the ai tend to just go straight for your lord so the pulling back can work well
also try to get a few conscript archers of your own using your starting money - or build a tower - and then after the archers are inside it break the steps up to it (make sure to put a burning brand in there to). That way they can fire out during an attack and the enemy can't rush them to kill them with meele troops - you can rebuild the stone steps after the attack to give them more archers.
A wall can also work well - look early on for any possible choke points and fill it with wall (after a tower) - it might not look very pretty, but early on looks are not important - you need defences.
If your playing the missions then keep up a defensive line, but make sure to remember the objectives - some of the attack timer ones don't give you long recovery times so you want to build archer huts as soon as you can - archers are key followed by crossbowmen for there armour breaking bolts. Archers can kiill armoured units, but they need time to do so (attack and then pull back to attack again works well as aside from knights the more armoured units are slow movers - barring the macemen who are the middleground troops

I actually found it way too easy once you discover some AI weaknesses. The enemy will always try to go for your keep and your lord if there's a way to get there ignoring all else. That means - if you don't mind playing cheesy - most missions become way easy.
As long as you do NOT close your defense, do not build a complete castle, but leave a gap in a wall. Or better, even - build a long corridor, two walls one tile apart with towers here and there. Build loads of archers and crossbowmen. Put those on towers. None of your enemies - no matter how strong - will get past that, and will try to rush in, regardless.
Especially once you get traps. Fill the corridor with traps and you are completly set.
That the AI does that mistake is even more disappointing given that it will attack buildings outside your castle walls if it can't reach the keep. Which shuts down your economy and makes your life way hard - and, because of that, just not closing the path turns the game from a challenge into something way too easy.
-Mnemon

I just wish the ai would make effective use of seige engines more often - sadly they tend to only build a few catapults and the odd tebuche (SP) and don't hold back their troops much when attacking (When they do hold back often as not they are sitting right in line of archers from the towers to get picked off).
That makes defence rather easy once you have a decently thick wall and a good load of archers and crossbowmen on the top. Though the ai tends to always attack at the same time - so 5 or so opponents attacking all at once can be a real challenge to counter!

I have to agree as well. I played Crusader long before I tried my hand at the original and I found that at the easiest difficulty level, it was ludicrously difficult. I think I quit on one of the first missions where you have to create a mixed group of military units to stave off an invasion and I might have had the time and resources to churn out 5 or 6 and my guys got slaughtered every time. This is why I don't understand who the target audience was for that Stronghold Extreme pack that was released last year.
Having said that, I'm managing to hold my own in the single-player campaign for the original game. I haven't picked it up in a while, but I've defeated two or three of the opponent lords at this stage.

This reminds me of Outlive (a brasillian RTS that mimics Starcraft a lot). I could never beat the AI in skirmishes. Once I figured that if I attacked extremely early in the game the AI wouldn't be able to handle it, I could beat it on hard with ease.
As for stronghold, I'm still playing the first one's economical campaign. Somehow it captures my interest a lot more than the military ones

What actually annoys me are the not-so-random events that keep fucking everything up. Like thieves stealing food, bandits coming out of nowhere, etc.
But I can still bear them usually.

Thing is if it were not for the time limits and the random events there would be no challenge to these modes - be better to just start up free castle builder mode and set yourself a few targets. Infact I wish we could triggor attacks and such in that mode - since one you build the castle and get income up to standard just what do you do?

1. I know, that's why I said earlier that's the challenge in those levels; I just think time is set too tightly, forcing you to rush to get the goals, and thus you don't have time to really enjoy creating a thriving economy. Oh well.
2. I do so like building in free build mode!
Also, you CAN trigger an attack there- hit F1 and call an invasion force.

Life just got a whole lot better :)
I guess its because I hate reading PDFs and can nomrally work out game controls, plus most manuals these days are so thin in details that they are not worth reading!