Too, I misjudged the Celts:

My regular warrior redlined the Gallic, then died. Doh. A couple of turns later, I retake Kyoto, then make peace, which nets me a discount on Chivalry and Monarchy. The Celts have just signed their own death warrent. Just not...yet...

5 turns after Richborough's temple expands, then it's ti...you need 1040 gold to upgrade your horsemen to samurai? Oh dear. The important thing to remember, here is that samurai don't need horses. And half the continent hasn't got horses. Persia. Aztecs. Friends. Give me money. Thank you.

Bonus:

And I'm wondering why my rearline cities all of a sudden suck, when I realize - ENTREMONT is my new capitol! Ha!

Osaka starts up the Forbidden Palace immediately.

In 630 AD, Brennus comes wanting to renew the peace deal. I pause for a moment. On the one hand, I now have 6 samurai, plus about 10 horses, which ought to easily be enough to take Alesia off of him. On the other hand, I'd much rather have all samurai, but I know it's going to take quite a while to get there. Choices, choices.

Fortunately, my declaring war kills all of my trade deals. Smooth.

Also fortunately, the Celts have guns. Oh dear.

Our glorious samurai have something of a hard time getting started against Alesia's giant gun defenses. Sooner or later, though...

It takes a lot of pain to do it, but Alesia eventually falls.

The good part about all of this is that Alesia was totally frontloaded - Lugdunum, their new capitol, has all of one musket defending. Camulodunum has two, but no matter - the tides of destiny carry my samurai into the city and an utterly defeated Brennus makes peace in 820 AD. The remaining Celtic possessions are scattered through the warzone of the north, cut off and unlikely to pose any sort of threat in the future.

About 940 AD, things start moving pretty fast. One, my Forbidden Palace comes in. Two, the remaining pikemen I need for a defensive line come into position. Three, my samurai have all arrived down south. Four, my deals run out.

It's time for a new war.

How's it going, royal ugly dudes?

Brennus, of all people, sells me gunpowder for 740 gold that same turn.

Also of note, London completes Copernicus. This is besides the Great Library and Sistine's. And London is a mere two cities from the front lines. You can see where this is going, I'm sure.

The massively fun thing here is that England not only DOESN'T HAVE ANY IRON, their saltpeter is on a mountain that isn't roaded. And I have, count them, 20 samurai. Hastings, for instance, a size 12 city, has a whole regular spear as a best defender.

The results are predictable.