Just place blocks with cannons?

Also you can simulate barbed wire with cobwebs or berry bushes.

The Problem with that being is that berrys dont grow everywhere and sometimes die and also require to be placed on grass blocks which isnt as a big deal but messes up some fort's aesthetic

Cobwebs don't die and don't destroy forts aestetich due to being placeable everywhere

this "Bunker" of yours would be used by the defenders to make their impenetrable wall even harder to breach.
barbed wire would be literally spammed on the battlefield, making movement hard and making cannon spam even more of a viable strategy

fair enough berries would probably be a better alternative considering theyre already relatively balanced besides for the biome limitations

having a movecraft type for bunkers to boost block durability and allow different cannon types wherever is an extremely easy to exploit to anyone's will addition that would give further defender advantage as stanchez mentioned. Plus, these would have to be fairly small and simple to build or usable within peaceful claims to be of any use whatsoever to an attacking force as it would be incredibly easy to shoot down as a defender or destroy in off-hours of a war
barbed wire sounds like a "cool idea" that will end up being either useless, a weak nuisance, or an extremely powerful damaging block, that in all cases will be spammed everywhere and once again add to the defender advantage as they can be placed strategically ahead of time. Not factoring the cost of the barbed wire which would, no matter what, disproportionately favor already large forces. It sounds like another element to make sieges annoying and tiring to deal with

Not considering the comments above, this would be an extremely banger of an idea, if we didn't want to limit Wilderness cannons to only specific types when they are stationed by themselves in the first place. However, as mentioned this would be pretty unbalanced if you could just build one in your town and use it. Maybe it should only work in Wilderness.

Cool idea though imho. But there are a few problems with it.

Every craft has a limit, of how many blocks it can or cannot have. Maybe to keep it simple have it so bunkers can only be a certain amount of blocks.

That way if you tried to put it on a wall or something it would merge with the wall becoming too big for you to even have it there.