This page clarifies some misunderstood rules in the Cities & Knights rules.
Elements: Elements are the resource cards of wool, ore, lumber, grain and bricks.
Gold: If you possess a town or city at a gold field and your number is thrown, you get to pick one or more elements of choice (no commodities!).
Obstacle: You cannot build a street by hopping over an obstacle (town, city, knights) of an opponent. The street must always be in direct contact with one of your own structures.
Trading Route: Your trading route will be cut in half when an opponent builds a structure in between two of your streets.
Street/Boat: You cannot connect a street to a boat without a town or city in between. Note: Your longest trading route doesn't count when a street connects to a road without town or city in between.
Discover Fields: If you 'discover' a hidden field, you receive the corresponding element (with a gold field you get to pick one by choice).
Discover Islands: If you place a new town on an island you haven't build any other towns yet, you receive a discovery chip (1 victory point). It doesn't matter if someone else discovered the island before you.
Street Building card: With the street building card you may build boats as well as streets.
1 per turn: You may normally only use 1 development card per turn. Two exceptions are A) victory point cards (may always play) and B) When you can reach the victory point limit in this turn. You normally play your card after you rolled the dice, Knight cards may also be played before you roll. Victory point cards and Knight cards will be placed open on the table, other cards will return to the bottom of the deck.
Wait a turn: When you buy a development card, you may play your development card in the next turn or after. You are allowed to keep as many development cards as you wish.
Victory point cards: You need to play out your victory point cards for them to count. Not playing them out directly may be a tactical move, but can also be risky when someone else is about to win the game.