Of course, there 's more to a map than just the brushes it contains. A map with just brushes is like a movie with no actors: It’s a still life. In order to have weapons, enemies, and even lights, you must add objects to the map that tell the game engine where to place these things. A map cant even be started if you don 't designate a player start point in the map.
This section deals with adding objects like the above, as well as other types of props, to your maps. Most of the objects mentioned above are referred to as entities. An entity can be defined as an object that is defined not of brushes but by code (either in the game or the engine). The half-life engine has certain classes of object that any MOD built on half-life can use.
Most Mods also define their own sets of entities, some of which are brand new, and some of which are subclasses or children of the engine classes. A subclass incorporates most or all of the behaviours and attributes of the parent, but also adds some new behaviours of its own. In the case of most LITH Tech games, for example, you can add a Door (“Make this brush move.”), you can add a Door subclass Rotating Door (“Make this brush move and turn on a point.”), or you can add a Rotating Door-subclass Destructible Rotating Door (“Make this brush move, turn and explode when shot.”). Each of these subclasses takes the properties and behaviours of the object above them in the tree (their parent) and extends them in new directions. You can still use the root classes in cases where you don 't need the added properties of the child objects, and often will.
This tree system is easy to see when you look at the entity list inside a half-life game. For example, each of the AI classes derives from the root class, but each type of enemy, civilian and major character is its own subclass, many of which have their own subclasses for different armies or different versions of the character. Such as a grunt, or a grunt realer, in Half Life SP.