Before the release of The Elder Scrolls Online, it became known that the capital city, the Imperial City, would keep its gates shut upon launch. The reason given was that the city had been conquered and occupied by the hordes of Molag Bal. It is now up to you to drive them out of the city and gain control over it, but that is not so simple.
First of all, you must manage to get into the city at all. To do this, your alliance must control most of the 18 strongholds in Cyrodiil. Then, and only then, can you enter the Imperial City through an entrance in the sewers. There are 3 separate entrances, one for each alliance, but only one will be open at a time. Which one that is can change quickly.