Most of haven right now is either charging down with 3 palibashers and tear everyone apart with a sledgehammer, or camp for 20 hours all night with your mountain dew with a bow guarding your ram.
Now i propose one thing, some new siege styles, let's face it, battering a brick wall isn't the same anymore, yes?
I would at least like to put the effect of mining down on a village, i mean if we mined out a HUGE area the ground would be unstable and cause the palisade to be extremely damaged, possibly destroying it, i propose we can collapse palisades under the pressure but however, brickwalls could be MUCH harder, to prevent digging in and raiding from the minehole, there would be a HUGE strength requirement to break through to give the element of surprise, but there would have to be a warning symbol & this would be a very risky trip, with the possibility of palisade remains dropping on you, brutally injuring you.
I also propose makeshift holes, but this would require a large amount of time, energy and strength, maybe lasting 30 minutes, but i think maybe from layer 5 you could assault using makeshift holes, with no sign of them coming, it COULD work, but once again, a hard tactic.
The only way to make your walls more resistant would be making large support nests, which of course could only be destroyed by underground rams, so whenever it be causing havoc from ground up then sieging from there, it could also be possible to cause large caveins from layer 5 if they have dug that low, it could cause problems on layer 1 so damaging the above would be nicer.