COC 6th Builder: How to unlock B.O.B?

Each player begins his/her Clash of Clans journey of building a village, collecting and investing resources, and climbing ranks with a single builder. They unlock their second builder for 250 gems by following the game's tutorial. The quest for unlocking furthermore builders continues with each additional builder requiring double the number of gems than that of the previous: 500 gems for the 3rd, 1000 gems for the 4th, and 2000 gems for the 5th.