From Dragon Eye Atlas
The Fighters’ Guild is a lose organisation with guild halls in capitals and other large cities as well as near borders, hotspots for adventuring and along trade routes as most of their members are either adventurers or mercenaries.
It provides its members with weaponsmiths and armour repairs where none such exist. Of course in cities with established craftsmen it does not compete with them, as the very purpose of guilds in medieval society is to prevent competition and keep prices high.
It also teaches fighting gambits, with a focus on the practical rather than the theoretical, though most guild halls do have a small library with books on fencing, warfare and other topics.
The Fighters’ Guild is the smallest of the organisations in this chapter. It also has the worst reputation, as a horde of ruffians and swords-for-hire, only slightly above bandits and criminals. That reputation is entirely unearned, or in some cases the result of individual misbehaved members, but the guild does not care much about its image. There are local exceptions, of course.