Airports MUST have clear glide paths to and from the runways. Mountains, tall trees, buildings, and other obstructions are not permitted at either end of runways and must be a functional distance away from them. Admins/ ops will have the final say in what constitutes obstructions!
Class A Airport: One or more runways which are 15+ meters (blocks) wide and 200+ meters in length. Airports are made of proper tarmac (asphalt or concrete) and are properly numbered. Runways should be numbered as “01” if a landing airplane would be facing North on approach, “18” for a South approach, 09 for an East approach, and 27 for a West approach. These numbers must be presented right on the runway itself. Take a look at the airports in New Terra City, Origin, or Port Of Hymns to see examples of Class A airports. The airport must also have a terminal and a control tower. Hangars are optional.
Class B Airport: One or more runways which are 15+ meters wide and 200+ meters in length, and made of a hard material (stone, sandstone, cobblestone, clay, or brick). Buildings are optional but highly recommended. The airport must have a control tower.
Class C Airport: One or more runways which are 15+ meters wide and 100+ meters in length, and made of any material (chiseled dirt, sand, gravel, etc. are acceptable). These runways are meant to be used by bush pilots and small aircraft in general. Large aircraft (airliners, bombers, heavy cargo aircraft, etc.) are not permitted to land at these airfields. Control towers and all other structures are optional but highly recommended.
Other Considerations: Airports can and usually should have places for airships to land and be parked in such a way that parked airships aren’t going to obstruct runways.
Seaports should obviously be adjacent to the sea, and positioned in such a way that two large ships can enter and exit the port safely. Building inside a bay is fine, but try and remember the two-passing-ships rule.
Dock areas where people walk should be 4 or 5 blocks (5 highly recommended!) above sea level to accommodate standard ships, with ladders reaching into the water. The docks must be well-lit and monster-free at nighttime. The harbor area should have enough open water nearby that excess ships can anchor offshore and row into port/ onto shore. One or more drydock facilities are highly recommended for shipbuilding and ship repair purposes.
Class A Seaport: 6 or more docks for ships.
Class B Seaport: 4 or 5 docks for ships
Class C Seaport: 3 docks for ships
Class D Seaport: 1 or 2 docks for ships