From The Battle for Wesnoth Wiki
the [era] top level tag
This tag describes one era. An era is a set of teams to play in multiplayer. In the multiplayer game selector screen, an era is chosen by the host by the 'Era' option button.
The following key/tags are recognized for [era]
- name displayed name of the era.
- id ID of the era - must be unique. No gameplay effect
- [multiplayer_side] a faction in the era. This tag contains many of the same keys as a [side] tag (A description of the [side] tag can be found in SideWML). When a multiplayer game is played, then the [side] tag for the scenario is merged with the keys(currently, not tags) of the [multiplayer_side] tag of the faction which the side chose.
- name a description that is displayed as the option for selecting that faction. (see DescriptionWML)
- id faction ID - must be unique to your era. No gameplay effect
- leader a list of unit types. When this faction is chosen, the side can choose any of these unit types to be the side's leader (i.e. "Choose your Leader"). They will also have the option of having one of these types being chosen randomly.
- random_leader Template:DevFeature: if a random_leader (list of types) is present, it would use this list to instead to choose a random leader. If not it would use leader
- random_faction default 'no'. If 'yes', then when this faction is chosen, another faction will be randomly chosen instead. The leader will also be chosen randomly.
- terrain_liked an unseparated list of terrains (see TerrainLettersWML). On random maps, these terrains will determine which keep the side is assigned, attempting to put it near the listed terrains.