Mods

Glest is fully moddable, meaning you can expand and modify the game to make it feel like an entirely different game. Not only is it open source, allowing you to change core aspects of the engine, but it is more than just a game. Glest is a game engine which can read data in its installation directory to play it. This data is called mods, and is generally split into a few basic types: Techtrees/Factions, Tilesets, Maps, and Scenarios.

Engines
Glest is no longer in development, but its forks, GAE and MegaGlest, are still in active development, and have already expanded Glest with new features such as a master-server on MegaGlest or emanations on GAE. Because of this, some mods may not be compatible with the differing engines. Currently, MegaGlest mods function on GAE, but GAE mods cannot work on MegaGlest. Neither engine's mods work on regular Glest.

Techtrees/Factions
A techtree is a full set of factions with their own resources, damage types, etc. You can only use factions that all belong to the same tech tree in a game (eg: Magitech). A faction is the team of units that a player can control, and defines the units, upgrades, how they work, etc (eg: Tech).

Tilesets
Tilesets define how the environment looks. For example, they hold all the information about the environment, such as fog, how long days are, etc, as well as store all the models and textures, such as the model used to make the trees and textures used for the ground. Thus, they can effectively make a map feel entirely different. Tilesets are so far fully compatable with all engines of Glest. MegaGlest supports cliffs, which can use a custom 6th texture by adding that into the tileset (which does not break compatibility. It will still work on all engines). An example of a tileset is the Winter Forest tileset.

Maps
Maps are the stage or design of the level, namely the placement of the objects, players, resources, etc (ie: In the Forest, Mountains, etc); This is done entirely in the map editor, and all engines come with their own map editor. Maps using more than 4 players use the MGM (MegaGlest Map) extension, while standard maps use the GBM (Glest Binary Map) extension. All engines can play the GBM format, but only GAE and MegaGlest support the MGM format.

Scenarios
Scenarios are set game options that can include scripting to allow them to do things games cannot regularily do. For example, you can have a beat the clock scenario, or a king of the hill scenario, or even a campaign with a storyline. Scenarios tend to always be engine specific. Scenarios made for regular Glest work on all engines, but GAE and MegaGlest both have their own Lua functions which are not always backwards compatible. Scripting in Glest is done in the Lua language.