Besiege has an official challenge that is hosted monthly by a modder and frequently featured in the official twitter, here is the list of official challenges. You can still do the older challenge if you're bored.

All Challenge Edit

November 2015: Rube Gold Berg Challenge

  • Create an awesome rube Goldberg machine

October 2015: Animal Challenge

  • Make a majestic animal
  • Optional: Bonus point if it's a Cat

September 2015: Spacecraft Challenge

  • Create a neat space ship for zero-G with equal to or less than 75 blocks
  • Optional: Spaceship Uses Steam/Water Cannon as Propulsion.

August 2015: Crane Challenge

  • Create a functioning crane that can rotate and raise and lower something along the Z axis. Movement along the main arm of the crane is optional. (No Block Limit)
  • Optional: Crane is able to finish Zone 6, 12, 16.

July 2015: Drone Challenge

  • Make a drone with maximum 100 blocks!
  • Optional: Drone has a 4 main propeller in symetrical position.

June 2015: Maze Challenge

  • Build a maze. Bonus points for building it large enough that a miniature 4 wheel vehicle can navigate. (No Block Limit)
  • Optional: Maze Don't have to include 4 wheels vehicle, only require a creative way to make it move.

May 2015: Design Challenge

  • 50 block limit, build something cool
  • Optional: Machine is Aesthetically Functional

April 2015: Automatic Challenge

  • Complete all levels with fully automatic vehicles. (No Button Pressing, Within Boundary, No Block limit)
  • Optional: Don't use Automation Mods

Alright starting it up again 2017

May 2018 Speeder challenge

Create a hovering or flying speeder zero gravity no mods

less then 100 blocks