Arrr pirates!! Here I be making ye big expert on the ROYAL CANNON! Me pirates love our cannon - it's so big and smashing!! hahaha! :)
So, apart from being big and bad the Royal Cannon does not actually give any bonus to attack or defense. Ye figure that?! Darn surprising that is!
But apparently the Royal Cannon be sinking some random imaginary ships passing by, letting our pirates go pick up the scraps, and due to this it be giving us a 5% bonus to our gold and lumber production starting from Level 2! And on top of this ye can upgrade the Royal Cannon up to level 6 and be getting a bonus of 25% to yer gold and lumber production!
Level 2: 5%
Level 3: 10%
Level 4: 15%
Level 5: 20%
Level 6: 25%
Now that be something good, but there be also some bad news with that. It be taking a whole day for just one level of upgrade and if ye get raided then the cannon is smashed and although you can repair it for free ye gotta do again all the upgrades. So for those pirates who are super busy with raiding and getting raid better be hoping for more like 5-10% bonus at most.