The Iron Cart Lock

The mine shaft narrows into a long, ribbed tunnel of black stone veined with rust-red ore, where the air tastes of grit and hot metal. Mine carts sit on split rails under a ceiling of creaking timbers, their wheels ticking softly as if the rock itself were breathing. At the far end, a heavy iron door glows with dim rune-light around three empty sockets, while nearby ore piles shiver in response to the door's magic. Each crash of metal sends dust drifting from the supports, and the elemental iron shapes stalking the tracks glitter like shards of broken furnace-slag given a hateful life.

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Puzzle EntryMagnetic cart-and-weight mechanismLevel4Hard

The Iron Cart Lock

4LevelHardDifficultyMagnetic cart-and-weight mechanismType

LevelLevel 4

DifficultyHard

TypeMagnetic cart-and-weight mechanism

Components5 parts

Time Limit6 rounds after the iron guardians fully emerge, or until 4 Collapse Marks accumulate, whichever comes first.

Primary DCDC 21

Quick Reference

Type

Magnetic cart-and-weight mechanism

Difficulty

Hard

Level

Time Limit

6 rounds after the iron guardians fully emerge, or until 4 Collapse Marks accumulate, whichever comes first.

Primary DC

DC 21

Checks

Crafting or Arcana18
Recall Knowledge to recognize the rune-door as a magnetic safety lock and identify that raw ore weight and cart placement matter more than brute force.
Crafting21
Build, brace, or re-route the mine carts so their weight and position match the door's locking pattern.
Athletics21
Force a cart onto the correct rail, wedge a brake, or haul a jammed cart into place under time pressure.
Perception20
Notice ore-color marks, floor grooves, and soot scarring that reveal which carts belong in which rails.
Thievery21
Bypass a final catch or release pin once the cart alignment is correct.

Overview

Discovery

The party can discover the correct pattern by examining the door sockets, the cart rail markings, and the plaque on the control pedestal, or by testing how the door reacts when ore weights are shifted among the carts.

Puzzle Handout

what the players see

Read the plaque: "Weight first. Breath second. Let the red rest left, the gray rest right, and the center stay empty until the mountain stops singing."
Check the three cart labels and rail grooves. The left spur is stamped with a red mark, the right spur with a gray mark, and the short middle rail is unmarked.
Set the red ore cart on the left spur and brake it hard.
Set the gray ore cart on the right spur and brake it hard.
Leave the center cart empty unless you need a balancing weight for the final step.
When both outer sockets glow, pull the flood lever to ground the lock.
Immediately pull the release bar. If the carts are correctly placed, the door unseals with a low iron groan.

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