Wars at the very begining usually are a bad idea :
There is approximately 20 provinces per nation.
If you spend soem ressoruces in a war, you'll face more opposition than agaisnt independents (so you'll be a bit slower), and you don't be able to siege and breach the enemy castle that early (if you manage to "win" the war). This means your enemy will defend his provinces, maybe attack you too, and take back some provinces.
Meanwhile, the other nations around you will still expand normally, and will even get some provinces that would have been yours if you didn't go to war.
Oh and you got the King of the City, that early ? That's a nice one. He's a little weird to play (the 80 popkill and the 8 unrest increase per month means you may not want him in your capital), but he seems really powerful. HopFlash (Pangaea in our game) had him in our last game, so you may want to ask him how he used him.
Little thing to note : He's good at expanding your dominion and nice at blood-hunting (getting blood slaves to sacrifice, while increasing unrest in a province). You may want him to lead your armies, and he's be useful at claming throens, too, as only him, your prophet, and your pretender will be able to do that. Also, if you make him your prophet (possible 6 months after your current one dies), he'll give you access to Holy4 spells, including a massive moral booster (fanatism), which is quite good for big battles. but either way, don't sacrifice your current prophet now just for that : having your prophet and him means you get 3 smiters, qhich are really good against independents.
PS (wow 4 posts during my writing) : Don't send him alone. He's a good army booster, but in no way is he as good as a dragon.