I love this idea, but for razing should be used special "Ballista" or "Juggernaut" ships, like on AoM. You'd send few of those very slow ships, maybe just slightly faster than CS to "Raze" attack enemy city and when they arrive they should stay there for 1-3 (depending on server speed) hours to raze few buildings. Each Juggernaut would destroy only one level of building in that city, and you'd have to choose before attack which buildings should be targeted, like senate, warehouse, farm, but not walls (so catapults remain important in game)!
Those ships would have weak defense, like maybe 40, weak attack (80), very slow (speed 6) and would be unable to destroy walls.
They would be costly, 6000 wood, 3000 rock and 4500 silver, would take 20 (or even 40) population as those are big ships.
Also, you'll get option to raze cities as soon as you research those ships, so you don't spend too many RP on that. They would cost 8 research points and you can only research them in lvl 28 academy.
Those ships would be able to attack any of your cities, even your first one, but your first (or only/last city) would be protected of demolishing your buildings below level of levels of buildings in newly founded city.
And I assume players wouldn't use those ships on small players as they would need cities to conquer, they would use them only on very experienced players to cripple them. I play few other browser games where there is "raze city" feature, so I know what I'm talking about.
I hope you like this idea
p.s. sorry for mistakes, my english is not very good