SteamWorld Build

You’ll earn this achievement with regular progress, so there’s no need to be focusing on it.

You can place down Miners straight away, as you enter the mine.

To place down Prospectors, you need to reach Milestone 6 (70 Engineers).

To place down Mechanics, you need to reach Milestone 7 (120 Engineers).

To place down Guards, you need to reach the
2nd level of the mine, which can be done by finding the broken mineshaft and repairing it. From my experience (combination of playing the game, and watching other people playing it), it’s usually bottom left.
There’s a door preventing you from accessing the area. You’ll need to follow the wires (yellow, blue and red) connected to the door. On one side of the wires is the door, the other side is the switch to activate the wire. You need to activate all three wires to open the door.
You’ll also find your first rocket part there.