Also, why would scouts spot the siege immediatly?
Alright it is an exageration... I mean't rather quickly as in the scouts (who are up ahead) would spot the seige, run to the carrivan, and shout "Hey enemies!". Should the scouts be killed the Carrivan won't get their scouts back and should thus run away.
Basically what I mean is, except for a rather small carrivan or one so large that it has its own private army (which did often happen). I don't see them getting attacked by a siege camp.
Goblins, sure, but what about all the babies they've snatched?
PERFECT!!! It could also be the method in which Goblins disable your traps. (Send a fake migrant who writes down the locations of all the traps... Escapes... Then gives them the report)
It's an underground fortress, how would they even know where to dig?
Well several non-psychic reasons (though the game runs on its own fair bit of psychic powers as it is)
1) Tales from the fortress to the world including Laisons and merchants who visited said location
2) Possible experience (Perhaps some goblins got into the base and left)
3) Trial and Error mixed in with educated guesses: If you know where the entance is (which they do) then you know where their base must be.
4) Spoke to animals, Artifacts, and other magic: Being a magical world there are quite a few methods of gaining a layout of your fortress or at least a vague understanding of where things are.
5) Underground Sonar: Also known as Tremor-sense.
6) Fake Migrants, Hidden spies, theives (who can sneak well).
7) Captured Prisoners: Ok this one makes me mad... It should be more possible to take people and creatures alive in battle.
Don't get me wrong I am not suggesting magica
I can't fix the ruptured waterworks after the psychic goblin tunnelers dig into the bedrock to get into my apartment complex or refill that pit they dug into the soil to get under my walls
After requiring flood control it should become easier. After you close it off you then pump the water out. I do agree that this possibly should wait until the ability to dam (and use pumps) are easier. Though some maps it is justifyable (Volcanos for example are supposed to be EXACTLY this kind of nightmare in the future)
learn to spell
Not likely to happen. My current ability to spell words come from years of trial and error. You are going to notice tons of spelling mistakes and there is nothing I can do about it without delving into the realm of the unreasonable.
Also I substitute words and forget letters for some odd reason while writing. You should notice that a lot.
the caravan guards have high attributes and are quite adept at killing multiple goblin ambushes at once
If the Carrivan feels that the guards can break the siege, which would require a report by the scouts, and that the location will buy their goods then they could, depending on their personality and relationship with the siegers, try to break the siege. A proper and prepared siege should be more then enough to deal with a simple carrivan "Eat squad of Marksmen Shoppers!"