They can upgrade, build, and rebuild structures, as well as upgrade your Heroes.
They can also remove Obstacles such as trees, rocks, bushes, etc.
One Builder can work on a building at a time.
Clicking on the Builder icon shows what upgrades they're currently doing. Clicking on an upgrade shows the location of the structure.
Always use your gems on builders first rather than resources or finishing an upgrade. Be patient.
Putting your Builders on the corners of your base may slow down the attacker and let you have a better chance of winning. However, it also allows them to be destroyed with single troops (if your attacker remembers to look for them), which may make the difference between a 50% victory and a 49% loss.
It can also help experienced players lure your Clan Castle defense troops to the corner to finish them off.
When the Builder is running, it moves as fast as a Goblin, and when walking, it moves as fast as a Barbarian.
The Builder does not always go to the same Builder's Hut it came out of.
If the player moves an inactive Builder's Hut, a Builder will be seen lying down on the spot where the hut used to be (but not actually sleeping). In older versions, the Builder is not seen lying down when the hut is moved; instead, there will be a dark spot of the Builder's shadow.
When the Builder is upgrading something, it takes him 1 minute to create 88 strikes of his hammer on the building.
If a Builder's Hut is placed on another building and you go to Village Edit Mode before it moves automatically, then hit edit on your active village, then hit cancel, then exit edit mode, the builder will appear to sleep outside of its hut.
The appearance of the Builder is similar to that of a Miner.
1st Builder Hut - Given for free at the start of the game, without any purchases
2nd Builder Hut - 250 gems (However you are forced to purchase this builder)