The internal calculation of village borders is mysterious and subtle. A short guide explaining how they work (prosperity is irrelevant).
Village Borders: How Do They Work?
- Prosperity doesn’t matter, only time.
- Size of world and era length don’t matter either.
- Same biome as source village speeds things up.
- Tiles with plants or animals speed things up a bit, if they’re not natively dangerous.
- Distance slows things down.
- Danger slows things down.
- Mountains are extra-slow (oceans are not).
Overall danger level can make villages contract in ways outside the scope of this guide (because I can’t find any numbers on it).
The basic time per new tile is 200 seconds, but there are two multipliers applied:
- Bonus term: this can range from 0.6 to 1.0; -0.3 if the tile type matches the village, -0.1 if the new tile already contains a plant or a non-dangerous animal.
- Penalty term: this starts out as 1.0 and increases without cap. It increases with +0.5* distance and +0.1*danger (of the new tile), and +0.2 if the new tile is a mountain tile.
There will be two tiles that can be expanded to; one on the left (counter-clockwise) side and the other on the right (clockwise). Whichever is cheaper will be expanded to first, ties break to the left.
So generally you can expect the first two border expansions to come at 120-140 seconds and then 240-280 seconds after the village formed. It will quickly slow down after that; the next pair will be 50% more each (180-210 seconds apiece), then 100% more (240-280 seconds), then 150%, etc.
If you want a large village (e.g. to get the 8 ocean tiles challenge) in a shorter Era, it makes sense to have it grow as a monoculture of the same biome and add mountains, oceans, or other primary biomes in later on. E.G. If you have a forest village and try to entirely flank it with ocean, it will take 28 minutes to grow into 8 forest per side and then drop it into the water, vs. 46 minutes to try to grow into 8 ocean per side.
As another comparison point an optimally-growing village (which yours isn’t) will cover 16 squares total by 30 minutes in.
Stray piece of advice: If you want to grow to cover mountains, the later the better; the mountain penalty is added onto the distance penalty rather than multiplied, so it hurts proportionally less later.