When a civilization learns Writing, it can begin to build its intelligence network. The first step is to build embassies in rival capital cities. That done, you can sign Right of Passage agreements and Military Alliances against third parties. When your empire has developed a sense of Nationalism, you can engage in Mutual Protection Pacts and Trade Embargoes against third parties. Embassies cost gold from your treasury to build, and diplomatic missions cost gold to execute. Build an embassy by double-clicking the capital city icon after you have learned Writing.