To unlock the SIMulated Town trophy in Cities: Skylines, you will need to play until you reach the Worthy Village milestone for reaching a population of 240 - 950 depending on which map you use. Once you reach Worthy Village, you will gain the ability to purchase a second map tile.
To unlock a new tile, hold
, choose New Areas and then click in the direction that you want to expand your city in. Once you purchase the second tile, the trophy will unlock.