to me right now walling everything off is kind of lame. but thats more because walls cant be damaged.
if in the future walls can be slowly damaged then it will be great because you have a reason to defend the walls as well.
Or completely bypassed by sneaks who climb right over them. Might be a bit OP given the size of ambushes and elven invasion late game, unless it would require a climbing check of a certain level, which would split it up. You could even go crazy with it and give archers the ability to bombard over the walls with accuracy penalties, if you wanted.
Casting your wall out of sea magma and cavern water, same thing, who knows if you'll have it full sized and fully intact by the time you need it? Well, similar thing... obsidian won't melt if you have an abnormally warm summer.
A cool idea, but wouldn't you need to build a wall of some sort anyway, just to contain the magma and water for casting?
Is there any way this could be implemented that wouldn't be completely tedious instead of Fun though? The enjoyment in seeing a tower you've spent seasons building crumble in a day is quite small.
Well, historically it is kind of like that. You spend years, potentially decades building a set of huge walls, the enemy spends a few months building siege weapons to negate them. Before siege engines (i.e. Troy, Athenian siege of Syracuse) it took forever. After siege engines (i.e. Tyre), it only took a few months. Long after siege engines (i.e. medieval Europe), the technology of wall building advanced to the point where it took forever, again. Ultimately you'd have to decide where the dorfs compare on the technological scale, and then try and adapt that into the game. Likely late medieval Europe given they have access to steel and plate armor, but prior to gunpowder so... yeah. No mortars, but highly advanced wooden constructions.
Ultimately they would need to just be strong enough to give you a chance to organize, equip, and slightly train a defense force. And leave stone behind so you could rebuild it, it's not like the goblins can powderize stone in the field.
Unless goblins know how to build ladders...