If the enemy walls aren't fortified too tightly and you have energy to spare, you can easily send in a few combat fabbers and have them reclaim the whole fortification. When defenses are more numerous, advanced ones work as well. If the defenses are only anti ground, you could even try air fabbers! This might be a tad OP actually...
If someone isn't smart enough to stick turrets right up against a wall, combat fabbers can just reclaim from a distance. Especially the Advanced ones. Though Infernos do a great job if you can get them in range.
Usually they don't make it in range before other ranged units destroy their targets. Their HP is nothing compared to the vanguards.
I like your style, I did exactly that today to this guy who'd turtled up behind a solid multi-layered box of defenses. His antinukes didn't cover the wall so I nuked the corner out and slammed a full army in far enough to take out some antinukes. Then of course some smart*** who went Orbital picks *MY* antinukes out with massed Anchors tanking for laser satellites...
Vanguards are amazing at destroying just about anything. They can take a Uber Cannon to the face and keep going destroying the commander in the processes. For them walls are not even there.
Walls are OP HP wise as they can singlehandedly stop any t1 warfare and with a turret an a couple of units they can even stop the combat fabbers. I don't mind them being strong but I believe their HP should be halved so that at least infernos can break through them. I have had games where the enemy put up a wall and 2 turrets and thus made my t1 tank arme redundant, so I had to tech up, but by that time they had put up more defenses and an anti nuke and I ended up having to throw everything I had (naval, air, ground, orbital and nukes and in massive amounts at that) to break through and win. The halleyable moon was used to secure another planet and we did manage to secure everything on the main planet that wasn't the walled in enemy base. We win the game, but it lasted 60 minutes longer than it should.
I still think Vanguards (and maybe any T2 tank) should be able to roll over walls like the giant tanks they are.