If you want to know which is the popular workshop of the builder of Clash of Clans, then you have to see this Mobailgamer post.
The Builder's Workshop is the main building in the Builder's Base, the parallel village that is unlocked at Town Hall level 4 and can be reached by using the broken boat that spawns in your main village by the sea.
It is the home of the master builder and is the target to destroy in an attack, as it is the equivalent of the town hall in the main village.
You must level up this workshop, up to the maximum level, which is nine, just like the town hall, in order to unlock buildings and defensive structures and start building a good village.
In this video you will see how to raise the builder's workshop to the maximum level: