Yes, there is. I made the mistake of not buying one, once. :/
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You need to buy the game to get a key...
No it is just the one time purchase fee for the game.
i think the most u would be able to do is dress like a bandit and loot a lot of caravans
You have to not be tied to a faction and simply declare war on a faction (attack one of their vassals under the demand option) and besiege a town and win. If you want more in depth go on the warband wiki google it
You can buy it from Steam. I would recommend buying it from www.taleworlds.com so they get more profit since they have an awesome dev. team.