This tutorial will teach you how to add a Barnacle to the game.
Create a test room. Insert a npc_barnacle entity and translate it's origin to the ceiling of the room. Switch the grid size to 1 unit and translate the entity in the Z axis 1 unit down. For some reason, the model origin is 1 unit below it's top face, so, if you don't translate it down, it will crop part of the model.
Compile and run your map, and you should have a working Barnacle, with sounds and AI.
You can download the example map here [].