The entity system is the infrastructure that runs the game's simulation/gameplay code. (The gameplay itself is not part of this system - the system is not even specific to RTS games - but it is all built on top of this system, and influences the system's design.)

The design is based most closely on the one presented in GPG5 (Bjarne Rene: Component Based Object Management; Game Programming Gems 5, 2005, page 25). Some other useful references: ​Evolve Your Hierarchy, ​Game Object Structure: Inheritance vs. Aggregation, Scott Bilas's ​GDC 2002 presentation: A Data-Driven Game Object System, and a few of the things linked from those.

An important concept in the entity system is the entity. This represents any kind of 'thing' in the simulation world - a person, a tree, a rock, an arrow, the terrain, and more abstract things like players and event triggers.

Entities consist of a set of components. A component is a largely self-contained piece of data and code, responsible for one part of the behaviour of an entity. One component might be responsible for rendering the entity; another for keeping track of its location in the world; another for tracking its health and reducing it when damaged and killing the entity when reaching zero.

Each component is an object instance in the C++ code. However, there is no C++ object representing an entity - each component is tied to an entity ID (an arbitrary integer), and an entity exists only as a concept defined by the set of components with the same entity ID.

The goal of the system is to ease development of moderately complex gameplay code, and to easily adapt to changing gameplay requirements. There is a focus on modularity and flexibility - it should be easy to read, understand, modify and replace a component. Therefore a component should be small, with few dependencies on other components. (These are often conflicting requirements - lots of small components require more dependencies than a few large components. Thoughtful design is needed.)