you dont want to go inventing new silly things like putty to fill in the cracks. i assumed that the rough constructed walls were like the ones you see all over the english countryside deviding fields up, made up of rough bits of stone carefully fitted together without mortar to make a very solid wall. i would be possible to climb something like that if you were good. but it would be slow.
walls made out of bricks, i imagine, would be like what the aztecs made. no mortar again, but so perfectly fitted you could not get a knife blade inbetween them. no way of climbing it at all, unless you have the modern suction pad things, which weigh something like 50 kg all up and require electronic control and all sorts of high tech non dwarflike stuff.
climbing walls in a frontal attack/siege is silly and rarely ever happened, unless people were trying to do it sneakily. i would have thought you would get the occasionall goblin master theif scaling your walls in the dead of the night to be kosher, but not armies. im sure anyone who spent their childhood in mock medieval battles complete with mums kitchen saucepans filled with mud on top of whatever ramparts were available would agree on this.
instead, siege towers (like caravans but with a few z levels, a retractable bridge on top and stairs inside) and ladders would be the go for getting into your fortifications. also, would be nice if they used their ladders as bridges for the occasional moat.
it would also be pretty neat if they built all this stuff at their camp outside your base. and if they did as the romans did and cut down some of your forest to stick a small wooden wall around themselves while they prepared, and hung around till you threw them off your land, that would be even better. as it is, i currently find sieges to be fun but extremely easy to put down. ive actually done it with 2 champions standing around a blind corner at the end of my main entrance. tricky flushing mechanisms and super awesome ballistas in bunker hidey holes are fun, but right now are not actually necessary.
some way to set fire to afforementioned siege towers would also be nice. as would some specialized goblins who would try swimming into your underground water resovoir to pop up out of the well in your dining room and cause havoc during a siege. teach you to use wall grates on even those entrances.
currently goblins are somewhat easy, but even dragons and things seem to die at the first weapon trap, or first hit from my legendary crossbowmen. matter of fact they either die so easily or get stuck so often in my moat through trying to destroy the floodgates in there not one has ever made it to my cage traps for capture. megabeasts really do seem to prioritize building destruction over everything else, often ignoring the hail of bolts and ballista arrows coming at them while they methodically smash every door and floor hatch in sight. if they can also take down walls that sort of behaviour would need fixing. and toady has said he wants dragons to get flying in over walls, picking up your dwarves and dropping them from high up, so wall destruction should not be a priority.