I'm on the third (IIRC) "Economy" mission trying to learn the ropes of the game and in addition to constant bandit attacks my whole castle catches fire every 5 minutes. Loving the game so far but this seems ridiculous.

I've been spamming wells around anything even remotely flammable and even then I regularly watch my entire orchard spontaneously go up in flames and get razed to the ground.

Of course then your food drops off and you lose about 30 peasants, which means they stop making bread and the food drops off more, and then your wells are unstaffed 5 minutes later when another massive fire hits...

Is this just a lame mission or is this a common feature in every game?

It's a lame mission.

Sometimes missions have a certain theme, and will challenge you according to that theme. In some missions that have you collect X amount of hops, you will get frequent hop weevil, but nothing really impossible.

I don't remember the exact map, but if you can, space out your buildings as much as you can so the fire doesn't spread that easily.

This level is ridiculous. Sometimes your whole castle is destroyed by fire and it seems there's nothing you can do to control it, even if you spam wells. But having a lot of wells is the only way to sometimes be able to control the fires, so you need to build them anyway. The problem with them is not the 50 gold they cost, but the peasant that is in charge of them and eating food.

I also find that walling the narrow pass near the signpost where the bandits come from is a good way to focus your defense efforts in one place and make a better use of the very limited land.

God I remember this mission. I was never able to build up enough food because the fires were so incessant. I must of tried this mission 10 times. It was too bad, I was really enjoying the game up until this point. I just played the other type of mission instead.