I really like this idea but a few issues that I think should be mentioned. The cannon can be destroyed, sure... but all you have to do to make it indestructible is cover the vital parts of the cannon and those blocks can't be destroyed. The only solution to this would be allowing blocks to be broken inside towns in siege zones... which just means you've got another fort (that ain't a fort) that will just be leveled by other cannons. I'm not quite sure how an issue like this would be addressed without simply allowing blocks inside siege zones to be broken by cannons, but that just creates a wave of more problems than solutions. Again, love the idea, but I just don't think it will work very well.

If cannons will be allowed in peaceful towns then the claims of the peaceful town should also be entirely prone to destruction because people can just make cannons with clever covering to make them invincible to incoming fire.

Maybe add a new block or craft called airstrike bunker that could protect the peacefull town. And make cannons not able to be near a airstrike bunker.

make it so the cannon cant work if blocks covering it

or just not add it at all....

Make shell detect if cannnon is within its blast radius even THROUGH blocks and apply the same damage to the blocks within the blast radius.

Example: i fire a shell. The cannon is covered with stone. The shell hits on top of the stone. Some part of cannon under the stone is within the blast radius (stone not calculated) the cannon is exploded by that shell even under stone. Done

This would be nice exept the cannon destroying part. But what if you could make maby pvp plots and those plots could have cannons but enemies can break them? Maby it could work?

This is pretty cool, but i agree with RuleBlocks, these sorta 'auxiliary units' would not be able to be covered, similar to that of the cannon integration protection on Dreadnoughts.