Labyrinth Lord is a retro-clone (a copy, to the extent permitted by copyright law) of the out of print version of Dungeons & Dragons contained in Moldvay's Basic and Cook and Marsh's Expert Sets. The material from the Basic and Expert Rulebooks has been condensed into a single volume, and even retains much of the original structure and organization. While there are numerous small cosmetic changes, like minor alterations to thief skill percentages and class XP progressions, most of the rules are simply rephrased.

The biggest changes are to the equipment list (significantly expanded and repriced, especially plate armor), support for higher levels (the charts continue to 20th level for human classes, instead of stopping at 14th), and spells (clerics get spells at 1st level, the addition of 6th and 7th level cleric spells and 7th through 9th level magic-user spells, faster acquisition of new spell levels, and no limit on the number of spells in a spellbook). The new spell progressions and spells are based on those in Advanced Dungeons & Dragons, not the versions in Menzter's later Companion and Master Sets.