New challenge: Dwarves start to get twitchy and paranoid from being surrounded by a hostile army for too long. Now you actually have to do something!
Yes, but this is a challenge that radically forces people to play the game in your preferred way.
Ahem.
Goblins will not always be able to be deterred by a small moat. In fact, at some point in the future, they may even be able to get through constructed walls. So no, "they're on the other side of the moat, what's there to be afraid of?" is not a valid argument.
The question of whether the dwarves are in danger or not is a matter for the player's defenses and the fortress. If the enemy can break through... then they'll break through, and there's no need for a hamhanded magic force that makes your dwarves go insane, because they'll go insane as a result of the enemy shooting at them.
On the other hand, my feeling is that it
should be possible for the player to at least aspire to build an unassailable fortress, one that no siege can ever break. It should not be easy (not nearly as easy as it is now), but it shouldn't hamhandedly be made impossible by making it so that when you have an invulnerable fortress, your dwarves still go insane from the non-existent pressure of enemies who can't hurt them.
Many people in this thread obviously like the idea of using armies and sweeping away invaders in a heroic charge. That's up to them. But that shouldn't be the
only solution. What you're asking for here is just that -- a mechanic to ensure that no fortress, no matter how well designed, is ever able to be (near-)perfectly defended against sieges.
I see that suggestion as being against the spirit of Dwarf Fortress. The game is not really, to me, about micromanaging dwarves or armies or anything -- it's about aspiring towards the perfect fortress, based on my own intentions and desires. I can try and make one that's structurally unassailable, or one set up for perfect productivity, or whatever... and nothing stops me from focusing on it however I want, and approaching the challenges however I want.
I don't think it should be as easy as it is now to make a perfectly-defended fortress, of course. Maybe you should never be able to
entirely reach perfection, only asymptotically approach it, keeping things interesting. It's boring if it's too easy to make that perfect fortress. But the effort of trying
is the game of Dwarf Fortress, to me.
This suggest is bad because it is a "Play the game my way or you're playing it
wrong" suggestion. The game should not use hamhanded rules to force players to solve things in specific ways -- it shouldn't say "You MUST kill the siegers or you lose, period!" That is not the Dwarf Fortress way. The Dwarf Fortress way is to present the siegers as a challenge or danger, carefully-simulated, and to let the player confront it however they want.
Or, to reiterate: If the enemy can make it past my moats and traps and devious defenses, then they can make it past my defenses, and will kill me. That's fine. If they can't, they can't. No other mechanics are needed.
Also note, of course, that siegers must have supplies and morale of their own, eventually. Pulling up the drawbridge and outlasting a siege is an entirely realistic and valid way to deal with it; that's the whole point of a fortress. The enemy should have various ways to try to get past your moat and so on, but you shouldn't be magically forced to rush out of your fortress and attack them -- that's silly and unrealistic.