Medieval fighting guilds and town street gangs?

[Zixinus]

During watching the webdocumentary on ARMA mentions "fighting guilds" and fencing schools. How do these worked and for whom would they teach? How comparable are they to today's martial arts studios/dojos/schools? Who funded these guilds, who managed them, did they have any codes of conduct?

There is also the question of how medieval (European) towns looked like and the question of crime. In modern day's crime organizations, the main income is from prostitution, car theft and selling illegal substances (alcohol in the past and drug of your choice today). What did medieval criminals profit from mostly? Did they often swore into the town's patricians (the owners of the town's land), being subjects to them or were street gangs independent? How were they organise and how did authorities respond to them?

[Akhlut]

I'm not sure about the fighting guilds, however, as to gangs, I imagine that prostitution was often employed, what with being the 'oldest job in the world' and all. I would imagine protection money and mugging was also commonplace, though I can't say for certain.

Further, bandits were a perennial problem for all medieval societies, as it could take days, weeks, or months just to mobilize against a group of bandits stealing/killing/raping passers-by on highways, much less find and kill them.

[Stark]

Extortion, robbery and fraud where big sellers even in the Victorian age. I'm not sure what the most reliable income for earlier criminal organisations would have been, however.

[Elfdart]

A very popular form of crime in medieval cities was the masonry mugging: Hoodlums would drop bricks or stones or blocks on people from the roof and then rob the victims.

One thing to remember about gangs, whether in ancient times, the middle ages or modern times is the fact that the biggest and most important benefit to being a gang member isn't the ill-gotten money, it's the defense they provide against other gangs.

[Zixinus]

What were the townies relations with the gangs? Could a careful tradesmen avoid muggery and live his life never knowing a single gang or were they permanently part of their lives?

[Zixinus]

Were there mills in the city and who owned them? In the country, lords owned the mills and they often forced encouraged their serfs to mill their grain there.

Because these mills cheated and were far away, peasents made hand-mills in their homes that were kept secret as it was punished. Would gangs run such a secret "underground mill" and profit by shares? This would be profitable because the peasant still gets more grain, cleaner than otherwise, that if he brought into his senior's mill? Or is there a block to this that I don't see?