Undermining/countermining seems somewhat represented by direct infantry attacks on walls already (I see @Grautvornix just beat me to it). Adding it would be worth it only if tunnels/terraforming are somehow added to the game, but this doesn't seem a big priority to me, all things considered. What really needs to be added/improved first is the ladder/siege tower unloading on walls, and the fighting on top of them, without making walls even more obsolete, and that last bit is the tricky part. For starters, ladders should make units climbing them more vulnerable, and siege towers should be extremely slow, but I think none of this would be properly balanced without wall towers automatically shooting, which is something that was removed in the past already, I guess because spamming them was too OP. My view to balance all this is that walls shouldn't include wall towers, but instead wall segments should be upgradeable to towers (I use this word for both current sentry and stone towers) that shoot automatically (imagining thermals would damage engines), for a more appropriate cost. Spacing limitations between towers would of course be considered. I know it seems this adds micro, but it really doesn't: one has to build towers behind walls anyway since wall towers are useless without a garrison, which is a waste of units and micro, and this should just improve what's already automatic. In conclusion: wall towers should be removed, while walls and towers should be better integrated, with towers and gates automatically doing anti-engine fire damage (additionally having attested catapult towers is something I already proposed), with garrisoning improving all this. Posted ranged infantry on the top of walls would shoot (like now), while melee infantry would fight unloading enemy units better.

@Asher Please see the latest release 1.9.1 in the first message of this thread (the onyl functional change). There are other changes inside as I am also preparing a second tool to prepare, support, manipulate and visualize Atlas maps.  The two tools post-atlas and the new one share common components hence the update of atlas-common 1.4.0 package (I actually should rename that to make clear this is not linked to the actual atlas tool in 0AD). I'll publish a first version of the new tool shortly in this part of the forum. 

Instead I would like to suggest walls that can be walked on (mechanics: moving garrisoned soldiers sideways replacing the next empty slot) towards the area of attack. Then, there could also be a successful attacker moving along the wall in the same way - garrisoning a wall segment from a siege tower or a ladder (and only from those) could be possible with a certain probability.

My amateur "historians" opinion:  Isn't the typical infantry attack on walls somehow representing kind of an undermining? (frankly, you may hit a stone wall for a VERY long time with a sword, a spear, or arrows, even with an axe, but without ny thing breaking ewxcept your own weapons.This is why city walls were so successful until the late middle ages, I believe.)

Perhaps it could unload them slowly. It would be cool to have battles on top of walls like in total war but that would take a lot of work.