Well, at the time my engine fired, the Stables were out, two Worker's Villages were left, and no other piles were below 6. And while I wasn't keeping exact track, I was pretty sure my opponent didn't have enough buying power to buy 3 Provinces or 6 Coppersmiths, so I felt safe building up on turn 12.
If my opponent had played better, then yeah, I probably would've gone for the green earlier.
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On another note: Would Chapel into a Coppersmith engine have been better? I wouldn't think so, but I remember that simulation contest where we found out that Chapel into IW/Island/Silk Road was better, so maybe?