Do become a Guardsman in the City Watch one can't just sign up, but has to be voted in. At least 50 citizens have to vouche for that person to be allowed entrance to the Watch.
The requirements to become a Senior Guardsman are even harsher. At least 400 hundred people have to vote for one for that person to get promoted.
Most members of the Watch are former soldiers. On one hand, it means that they have many combat classes, making it easier for them to stop fights. On the other hand, it means that they possess less levels in the [Guardsman] class, resulting in lack of better skills that would make it much easier for them to do certain parts of their jobs. For example, almost no one has tracking skills, making it much harder for them to chase criminals.