Limits of Unseen Engineers

Question

The Pathfinder spell Unseen Engineers has the following description:

Unseen Engineers

You conjure an invisible team of tiny engineers to construct a trap at alarming speed. Originating at your location, the team must remain within range of you or the spell ends. The engineers construct a mechanical trap of your choice. Construction takes a number of rounds equal to the trap’s challenge rating. The engineers must have the materials available. At the end of the construction time, use your Craft(traps) skill with a +5 bonus to determine the success of their work. If the engineers succeed, the trap is complete and the spell ends. If they fail, the materials are deposited at that location and the spell ends. If the spell’s duration ends before the trap is complete, the engineers automatically fail.

The engineers move at your base speed and can carry only materials required in the construction of the desired trap. They can’t attack in any way or be killed, and dissipate if they take 12 points of damage from area attacks (they get no saves against attacks).

What form must these required materials be in, and does this change for magical traps?

(Of course, this is ultimately up to my discretion as GM.)

Interpretation A

The caster needs only the raw materials (e.g. wood, stone, earth, metal, and/or specific poisons), and the Unseen Engineers do the crafting magically-fast.

Examples:

• A Barbed Wire trap would only require raw metal (or metal shrapnel), and the Unseen Engineers would make Barbed Wire out of it and place it as directed.
• A Bull Rush Statue trap would need the stone/metal to make the statues.
• An Acid Arrow Trap would require [metal? wood? acid?], and the Unseen Servants would craft the arrows and place them as directed.

Advantage: This interpretation circumvents awkwardness/complications related to magical items and extremely simple traps. (How would the engineers build a pit for a pit trap? Would they need to have a scroll or potion to create a magical trap, or would the caster be the one to cast a spell for that trap?)

Disadvantage: It seems too easy to make complex traps. Some items seem too complicated to be made this way.

Interpretation B

The Unseen Engineers serve only to move things into place. The caster needs specific, pre-made materials, such as....

• A pre-existing pit for a Pit Trap?
• Pre-made poisoned darts for a Poisoned Dart Trap?
• Pre-made barbed wire for a Barbed Wire trap?
• A pre-made portcullis for a Falling Portcullis trap?

Advantage: This makes it more realistic: the Unseen Engineers move stuff, which is why a bunch of them do it so quickly. Disadvantage: It begs the question of how to make more complicated things, like magical traps or traps with lots of gears, glues, etc.

Answer 1

The effect of Unseen Engineers is talking specifically about the costs to create a trap:

Mechanical Trap Cost

The cost of a mechanical trap is 1,000 gp × the trap’s Challenge Rating. If the trap uses spells in its trigger or reset, add those costs separately. If the trap cannot be reset, divide the cost in half. If the trap has an automatic reset, increase the cost by half (+50%). Particularly simple traps, such as pit traps, might have a greatly reduced cost, subject to GM discretion. Such traps might cost as little as 250 gp × the trap’s Challenge Rating.

After you’ve determined the cost by Challenge Rating, add the price of any alchemical items or poison you incorporated into the trap. If the trap uses one of these elements and has an automatic reset, multiply the poison or alchemical item cost by 20 to provide an adequate supply of doses.

What specific material components are those is left at GM's discretion, and could be anything, from a pile of raw materials, to delicate mechanisms, to all other the necessary tools for the job, as long as the total material cost is paid and spent during the process.

A simple Common Pit Trap is CR 1, and should cost about 250 gp to craft on materials. While a Poisoned Pit Trap is CR 12, and should cost between 3,000 gp and 6,000 gp. Considering that Shadow Essences cost 250 gp a dose, twenty of those would be 5,000 gp, so the cost of this pit should sit around a minimum of 9,000 gp.