CAPITAL

You WANT a 15c capital, but you can very nicely play with 9c too.

Let's look at some problems:

Try snag a 9c or a 15c quickly enough. Experienced people will snag it as their 3rd village or 4th village (while building the other two villages around that square and vigorously defending it), while less experienced and overeager people will snag it as their 2nd.

Let's see what you gain. Whenever I mention crop-fields, I always do so with a L5 grain mill/L5 bakery. If you don't have that...I can't help you. You should go play tetris.

A L10 cropfield gives 300crop/hour:

- 6c -> 1800 crop/hour
- 9c -> 2700 crop/hour
- 15c -> 4500 crop/hour

Let's look up to L15 crop fields, which are buildable with 2 warehouses.
They give 1200crop/hour each, so:

- 6c -> 7200 crop/hour
- 9c -> 10800 crop/hour
- 15c -> 18000 crop/hour

L16 crop fields, each giving 1500 crop/hour, requiring 3 warehouses are still doable:

- 6c -> 9000 crop/hour
- 9c -> 13500 crop/hour
- 15c -> 22500 crop/hour

L17 field has 1950 crop/hour
L18 has 2400 crop/hour
L19 has 3000 crop/hour (and requires 12 warehouses and 3 granaries.

AAAANYWAY. As you can see, a 15c with manageable L16 fields can maintain a 20k crop army with no problems (with no plus account which gives you another 25%).

All the big players are doing this, and doing it comfortably. There's the occasional dude with a 9c capital in the top bunch, but none will have a 6c, even though it is possible, but
not worth it. It IS possible, but it's not worth it.

What about the 9c capital?
It often happens that all the 15c are gobbled up, but you snag a few 9c with little problem. What should you do?

15c has a problem-it requires CONSTANT merchant control on it. This means that if your cluster of villages is far from your 15c capital, you will have pains. You cannot build on the meager resource production (a max of 200 from the field, plus the oasis's which you should only get if it's 25% crop with it, and the resource extenders, which you shouldn't build). So you will be forced to move stuff around a lot, which is far less painful than a big army on a 6c, but you have to do it. Gaul merchants help a lot, because they are quick and carry a lot. At least Teutons carry more, but roman merchants are horrific.
You don't want to stagnate. Build close-by support villages, and be aware you will sink money in it.
So, a 9c capital has a solid production. I won't bother analyzing the numbers, except in 2 cases, for L10 fields, and L15 fields, with no oasis, but with the brickyard type buildings:

L10

- 6c -> 1000/hour
- 9c -> 750/hour

L15
- 6c -> 4000/hour
- 9c -> 3000/hour

As you can see, not a really big change nor is the gain in crop, but that's far more useful than the gain in resources.
A 9c capital will need little support, although it does help to have a village funnel resources into it.

A 9c capital has a few more advantages: you are able to get one with little trouble, and a bit of scouting. People won't fight you over it, knowing that there are more around, and they are looking for a 15c anyway. This means that you can comfortably wait until your 3rd village to cap one, even if you start slow.

The second advantage: it's not an impotent village until you sink a lot of stuff into it. You can still have a strong economy in a self-sufficient, but slower, village, and start building an army alongside the economy.

Ok, so until now, you hopefully understand that you should go for a 15c, and if not, a 9c. A 6c can never really hold a big army.
If you are still unconvinced, think it through.