Scenarios

Scenarios are special files that tell Glest how to play a game. They have set values, so you cannot choose your faction, nor your map, tileset, and opponents. However, scenarios can use scripts to allow them to do things not otherwise possible, such as campaigns. Due to this, they are able to tell a story, and can be considered a more linear story mode compared to the "free play" of a custom game.

Engine Support
All three engines of Glest, the original one, MegaGlest, and GAE support scenarios, but all to different degrees. MegaGlest and GAE expanded on the original Glest, and thus have all its features and more.

Scripting
Scripting in scenarios is done with Lua. This is a simple and fast scripting language which has many pre-defined functions for Glest scenarios. For example, you can use createUnit('unit_name', 0, startLocation(0)) to create a unit named "unit name" for player 0 at the start location of player 0. See Lua for how to write scripted scenarios.