Nether portal

Cities can create a nether portal, with access to the nether through the use of a warp point, very similar to spawn's setup. Warp points that lead to the nether will lead to coordinates at a multiple of 10 of that city's location. For example, if a city was based at 3,500 X and 4,000 Z, then you can be certain that the nether portal would be placed in the nether near 35,000 X and 40,000 Z.

The following requirements are needed for a Nether Portal in a city.
 * 15,000 fens.
 * 1) 10 x 10 plot designated for the Nether Portal.
 * 2) An actual Nether Portal that is lit on that plot.
 * 3) 2 Signs
 * 4) 9 Obsidian

Observations:

1: Upon the creation of any Nether Portal, the signs are used for the warp points, and the obsidian is used for a platform on the other side. That platform has a small and restricted invulnerability zone to protect against instant death upon reaching the nether. Abuse of this invulnerability zone can provoke a jail, and even a ban upon continued abuse.