Step 1: Figure out the Concept

This step drives all the other decisions you’re going to make. It’s the most important, but also the simplest step—decide what you want the trap to do.

Types of Traps: A trap can be either mechanical or magical. Mechanical traps include pits, arrow traps, falling blocks, water-filled rooms, whirling blades, and anything else that depends on a mechanism to operate. Magic traps are further divided into spell and magic device categories. Spell traps are simply spells that themselves function as traps, such as fire trap or glyph of warding. Magic device traps initiate spell effects when activated, just as wands, rods, rings, or other magic items do. The rules for cost, CR, and construction differ depending on the type of trap you intend to make.

Elements of a Trap: All traps—mechanical or magical—must have the following elements: trigger, reset, Search DC, Disable Device DC, attack bonus (or saving throw or onset delay), damage/effect, and Challenge Rating. Some of these elements may be more or less important than others, and some traps may also include optional elements, such as poison or a bypass.