Natural Law

"While within city limits, you need to obey the city's laws."
There's a lot going on there. Where are the city limits? How do you find out the laws before entering those limits? If you break the law in a city, can you then leave and go to somewhere else, or can the city hunt you down?
To help answer these, the Valley has something called "common law". These are laws put in place and ratified by the High Council, and apply to all areas and nations within the Valley. These laws are broad and simple, the kind of things everyone can agree on. Or nearly everyone, as slavery is illegal, which presents a problem for the Sands. But by and large, the laws are pretty good, if sometimes a little lenient. Nations who are a part of the council also keep copies of their laws in Sandpoint, Skor, and Tsar, so at the very least people who're prepared to do some reading can check things in advance.
The downside to common law is it must require no specialized structures or abilities to enforce. One can't expect a travelling lawman to arrest someone, and then construct jail in which to hold them, or subject them to powerful magics to force them to reveal the truth. That means punishments are limited to execution, seizure of assets, or sending them off to a major city to be dealt with. For minor offenses, a letter may be sent to nearby settlements, warning them of the citizen's behavior.
Most infractions boil down to just hanging the perpetrator, which at least may deter some crime. The Common Laws are often summerised as:
"While outside city limits, all major crimes such as murder and banditry shall be punished by steel or rope."
However, there is another set of laws that govern the Valley, that most people don't know about, and couldn't break even if they tried. These are referred to as the Natural Laws, and govern the safety and continued existence of the Valley as a whole. A sect of druids (known colloquially as "The Natural Order") is in charge of enforcing these laws, and appointing agents to oversee particular areas as deemed necessary. 
Exactly what these laws are isn't clear. When Orcus threatened to merge the Valley with the Abyss, the Natural Order did not intervene. Nor did they act when Lady Macry tried to become the avatar of Muir and cleanse the Valley of evil in a wave of righteous fire. When a group of Aberdeenian miners tried to quarry a certain mountain, they were met first with dire warnings, then a bloody reprisal. Druids that attempt to teach druidic to non-druids, or who wear metal armor find their powers either wane or disappear completely, and The Natural Order is assumed to be the reason behind it. 
There are no writings listing the Natural Laws, and a lot of trial and error is needed to find exactly which places are "sacred" and which are fair game for development. Druids who come to their powers naturally are usually approached by someone from the Natural Order, and presumably at least given some pointers.
The greatest mystery is what the Natural Laws actually aim to protect. No one is sure what would happen if a concerted effort was made to break all known Natural Laws, but no one is really willing to try. The Natural Laws are often summerised as:
"Look, if a group of druids tell you not to do something, it's just not worth arguing with them. They're mean and cruel and weird, and there's plenty of other trees if you need lumber that badly".