As it is right now, all the "wall cannons" are simply underpowered.
One heavy tank can destroy probably 20-30 wall arty's if there are not so many people on the cannons at the same time.
Unless it is a 5v1 from the start, heavy tanks can just keep going around and destroying all the cannons on the walls.
Canister shots deal little to no damage to the point that using a gun is just better than bothering with canisters, despite them being "specialized ammunition" only restricted to a single cannon type.
I understand that the balance between defenders and attackers was among the first on the list of priorities to balance out the PvP, however obviously considering how much effort is given into building forts and designing good defenses, simply having no advantage at all is unfair.
It is very common to see tanks being used by defenders to defend a fort, simply because anything that is made with the purpose of being a defensive tool is nerfed into the ground, so sometimes (or perhaps most of the times) "leveling the playing field" by just going with tanks vs tanks in a defensive siege is a better option.
Meanwhile, the possibilities for attackers are almost endless. They can bring many tanks at once at the battlefield, or use wilderness howitzers from out of range, Fighter aircraft "swarm" is also an option, but they simply do not need to "get creative" with possibilities.
Bringing 1 fighter or 1 tank at a time, combined with as many sweaty PvP-ers as you can get has worked very well so far.
After all, back in the days when that did not work, "crying for nerfs" fixed the issue much easier than actually getting creative and good at the unique server mechanics.