I still have the same 12 towns that I had before the raised levels of version 8.8. Now I would not have corruption in any of them were I to build #13 (i.e. all GRs/Palace upgraded), but I've found little sufficient reason to rush this. Plenty of useful levels to build on those 12 towns... The best island for mill/mine levels near me, one on which I already have 2 towns, is Wine, so I could produce even more of the 1 good I'm not short of for the medium term. So, a "watching brief"...
I'm finally corruption free in my 13th town and it's finally producing a decent amount of wood/crystal after nearly 4 months from when I first planted. You say you have a 13th town ready to go, corruption free? Well let me suggest a productive use for that town while you decide what to do. Use it as a mobile barbarian pillager.
Here's what you do. Plant a mobile on a nearby island where you don't have any other towns. Then from one of your main towns, pillage the barbarian village that's on that mobile island. Pillage it to whatever level you want, and you can reset it back to level 1 whenever you want just by removing the mobile and re-planting. The barbs will never attack your main town, only the mobile, which you're not using for anything anyway. And they can only siphon resources from the mobile island, which again, you're not using xD. Never have to worry about retaliation and you also never have to pay a cease fire. Reset it as often as you want.
I didn't think of this strategy until after starting my 13th town, but I fully intend to do it with #14. Pillaging them to level 39 is really easy even without a forge, and that puts an additional 1.7m resources in your coffers. Minus troop losses and upkeep, which is almost nothing. It starts to become a little more expensive after level 39 into the upper 40's, but the rewards get bigger too. I would avoid level 50 at all costs however, unless you're going for the achievement (defeat lvl 50 on 3 different islands). They start using balloons and gyros at level 50 and it turns into a bloodbath.
But you can rake in an extra 4.7m resources from level 1 to 49. Again, minus troop losses and upkeep. It's still a healthy profit.
Decent/clever strategy, thanks, but not for me. Too much like hard work! I don't doubt it costs in, though.