Some buildings have additional Citizens you can unlock. With exception for the Smith, you can unlock them in any level after constructing the building. Most of them you will unlock without even trying, as most unlocks are pretty easy.

You get 100 extra units of supplies with every new level!
After building the Sawmill, the Farmer is unlocked by matching five tiles at once (Must be in a row, L or T shapes do not count).

Your Hammer will be filled more quickly!
This is the only "timed" unlock. After building the Forge, the Smith is unlocked by scoring 10.000 points during the next ten levels.

You get 200 extra units of supplies with every new level!
After building the Quarry, the Miner is unlocked by scoring 2.000 points during one level.

You get 100 extra units of food with every new level!
After building the Mill, the Miller is unlocked by making 15 successful moves at once.

You get 200 extra units of food with every new level!
After building the Field, the Peasant is unlocked by scoring 3.000 food-points during one level.

You get 300 extra units of food with every new level!
After building the Tavern, the Winemaker is unlocked by making two combos after another.

Your Plus bonus will be filled more quickly!
After building the Tabularium, the Scholaris unlocked by making three combos after another.

You get 500 extra units of food with every new level!
After building the Port, the Fisherman is unlocked by matching five tiles (Must be in a row, L or T shapes do not count) at once, three times in a row.

Your Shield will be filled more quickly!
After building the Colosseum, the Gladiator is unlocked by creating a chain reaction, consisting of 15 matches in a row.

Your Time Bonus will be filled more quickly!
After building the Pantheon, the Priest is unlocked by completing a level in less than three minutes.

All of your bonuses will be filled more quickly!
After building the Forum, the Orator is unlocked by scoring 5.000 gold-points during one level.