13th Age is an "open" d20-based tabletop fantasy RPG similar in play to games like D&D 3.0/3.5 and Pathfinder.
13th Age makes use of many game mechanics and features that are intended to develop characters and story as the game is played. For example:
- Icons - Icons are powerful factions that exert their will on the realm in some way or another.
- Relationships - A quantitative measurement of how much influence a given Icon exerts on a character.
- Backgrounds - Instead of advancing individual skills, a character assigns points to jobs or positions they have held, which can help them with actions in-game.
- One Unique Thing - Every player character has one unique thing about them that separates them from every other individual in the campaign's universe. This defines both the character and the universe by exclusion.
13th Age also eschews grid- and numerical-distance-based mechanics in favor of simplified range mechanics. Two characters are either:
- "Engaged" - In direct melee combat
- "Nearby" - Within one move away
- "Far-away" - More than one move away
Mundane combat equipment is based only on the class of weapon and the class of character using it. For instance, a basic attack with a dagger in the hands of a rogue does the same damage as a basic attack with a longsword in the hands of a fighter.
Speaking of fighters, martial classes gain some variety in their combat. Fighters, for instance, have features that allow their attacks to proc certain advantages depending on the roll of the die and if the attack hit or not. Rogues build momentum as they attack. Barbarians rage.
Critical hits are as simple as rolling in the crit range once.
Rob Heinsoo worked on D&D 4th Edition as lead designer. Jonathan Tweet designed during the release of D&D 3rd and 4th Edition. They are both very experienced game designers who have put their extensive combined experience into making 13th Age a quality game.