NOTE: THIS GAME MODE IS NO LONGER AVAILABLE IN THE CURRENT VERSION
Free build is a game mode where you are free to mine and build whatever you see fit. It is currently the only game mode where there are no enemies present. This means that the only way you can die is to take too much falling damage, or just fall into lava. Free Build was moved into Survival mode under No Enemies as of the 1.4.2 update.