Initially, the Weapon Lab is a large concrete semicircular structure on a larger circular base. The top is covered by heavy blue plating. A female villager in a lab coat and protective goggles stands next to the lab.
At level 2, the Weapon Lab gains a small blue overhang on the far back. The concrete base is outlined with blue plating.
At level 3, the rest of the concrete base is filled in with another semicircular structure. In the middle is a round metal balcony with a circular warning tape design in the middle. A ladder is added.
At level 4, the blue overhang gains a great deal of bulk, and now has a fan on the top.
At level 5, the blue overhang grows taller and has a drill-like structure on one side. There is a large amount of wire coiled around the place where the semicircular structures do not meet. A warning-tape design is marked on the front of the Lab.
The Weapon Lab is the only building whose build/upgrade costs unlocked at a certain Headquarters level exceed the cost to upgrade the Headquarters to the next level (by approximately 20%).
Although the level 5 Weapon Lab is unlocked at HQ level 19, its cost is set to as if it were unlocked at HQ level 20 (it would cost 4,400,000 wood, 4,300,000 stone and 3,400,000 iron if it is approximately 20% more than the cost of upgrading the HQ from 19 to 20.)
When a Prototype Defense's construction is complete, it will be covered by a brown tarp, so players visiting your base will not be able to see which Prototype Defense you have ready.
However, they can know that they have a new defense ready to go once the original one breaks down.