You need to upgrade your Warehouse twice to get this acheivement. Here are the required elements :
- 1st upgrade : 4x Wooden Beams + 4x Scaffoldings or Crystals
- 2nd upgrade : 7x Wooden Beams + 7x Scaffoldings or Crystals
Wooden Beams and Scaffoldings can be obtained :
- by opening workers' or reward chests (daily challenges and events)
- at the Notice board
- by buying the Crystal Tree (14,99$) -> gives 4x Wooden Beams + 4x Scaffoldings
You can also randomly get this achievement just by visiting an other player's world.
You can visit your friends who have the game or the members of your clan by clicking on the social button on bottom right of the screen.
To join a clan, you must have built the Clan Tower which unlocks at level 18.
Note : Coliseum events allow you to visit unknown players' worlds (click on the event logo under your crytals, Individual and Top players).