In the high middle ages, how did the growth of cities lead to the decline of serfdom?

 Believe it or not, towns actually increased the weakness of both feudalism and manorialism. Towns that became wealthy through trade. Since the towns were getting larger and larger, the lords and barons that ran the town began to fear what the towns would do if they had more power. Royalty began fearing that the ''Rise of the Towns" would be the time that they were overthrown.  
 I hope this helps.