My questions are more about diplomacy,

1. I'm making a mod that involves a hidden city (El Dorado). The civ labled El Dorado cannot build any units more settlers. Its highly restrictive. In fact all it can build is gold units (flag unit, victory point thingy). But i'm concerned about other civs tacking advantage of the single city civ. Is there anyway to stop diplomacy/war between the El Doradan Civ and other civs (except human player)?

2. When you first come into contact with a civ via contact with a unit which belongs to said civ but is under the hidden nationality you get the knew diplomacy contact in your diplomacy list and you are able from then on to negotiate with said newly contacted civ. Can you stop that from hapening? so the civ remains undescovered until you properly contact it via any of t he other methods.

cheers for any help

on point one, you could give the city some really strong, immobile, unbuildable units(maybe also with a strong 2 square bombard). This would make other civs think twice about attacking or pillaging territory. you'd also need to give it a wonder or two that generates a lot of culture so that it doesn't flip but not too much or it might start gaining cities by the same method. I don't think you can prevent diplomacy, though.

alternatively you could surround it's territory with immpassable terrain of some sort.

don't think I can help you at all with point 2. sorry.

I am not entirely sure what it is you want to prevent, but if you are worried that 'El Dorado' is discovered by player establishing an embassy then you could make a specific Government for the 'El Dorado' civ that is "Immune to: Establish Embassy". You need to make sure 'El Dorado' only have access to this 1 government and probably also that no other nation have access to it.