City raiding means forming a raid group and attacking a capital of the enemy faction. The goal is usually to kill an enemy faction leader. On even servers, city raids can be hard to perform, as the defenders of the capital usually outnumber the raid group.
Raiding the Alliance
The Alliance has four capital cities: Stormwind City, Ironforge, Darnassus and The Exodar. The most populated ones are Stormwind and Ironforge, while Darnassus and the Exodar contain few to no players at all.
Raiding Stormwind City
Stormwind is the capital of the Humans. Stormwind is one of the harder cities to raid. To get to King Varian Wrynn you have to go through the Valley of Heroes which contains a lot of guards, and the Trade District where the majority of players is located. You will most likely face tough resistance from the Alliance here. There is an alternative way into Stormwind, the Deeprun Tram. The Deeprun Tram is almost always used after raiding Stormwind or Ironforge. Stormwind Keep is very close from the Tram, so this is a good way to get in. There are areas of Stormwind that contain no guards, for example the catacombs under the Cathedral of Light. A raid can easily be summoned there to attack the city from behind.
Ironforge is the capital of the Dwarves and Gnomes. Ironforge is usually considered the main capital of the Alliance, which means that a lot of players is there. The only way into the mountain is on a long ledge full of guards which leads to the gates. Just inside you will find the square where everyone is. You will probably be attacked here. From here there are two ways to King Magni Bronzebeard, left and right. Get through, and the king's fortress is on the south side of the Great Forge. You can come through the Deeprun Tram, but it is a lot closer to the main square than it is in Stormwind. There are small unpopulated houses spread out in Ironforge where raids can be summoned.
Darnassus is the capital of the Night Elves. Darnassus is a rather easy city to raid. The only way into Darnassus is through a portal in Rut'theran Village on the coast of Teldrassil. The only way there is with a boat from Auberdine, Darkshore. You will be teleported to almost the center of the city. From there, get to the Temple of the Moon in the south. After a couple of stairs you will find Tyrande Whisperwind. Darnassus is a very unpopulated city because it's very far away from the main cities Ironforge and Stormwind. You will face little to no opposition.
Raiding the Exodar
The Exodar is the capital of the Draenei. The Exodar is also a very easy city to raid. To get there, you must get on a boat in Auberdine, Darkshore. It will take you almost directly outside one of the two entrances. Get down and Velen is just in the other side of the room. There is an another entrance, the main entrance. This one is almost never used because there are more guards there, and it's a longer way to Velen. The Exodar is most likely the most underpopulated city of the Alliance. An Exodar raid goes very smooth.
Raiding the Horde
The Horde has four capital cities: Orgrimmar, Thunder Bluff, Undercity and Silvermoon City. Orgrimmar is the main capital and is heavily populated. There are usually a lot of people in Thunder Bluff and Undercity as well, while Silvermoon is very underpopulated.
Orgrimmar is the capital of the Orcs and Trolls. Orgrimmar is the main capital of the Horde, and is very hard to raid. There are two ways in. The first is the main gates. There are usually a lot of players outside, dueling each other, and the people waiting for the zeppelins in the two towers. If you enter this way, you will find yourself in the Valley of Strength. There are always a lot of people here doing their buisness. From there you will have to go through the Drag until you finally come to Grommash Hold where Thrall and Vol'jin are located. The other entrance is the western entrance, the Talon Gate. This entrance is almost unknown by many, and not many players hang around there. All Horde players who use portals will end up in the Valley of Spirits however. Be aware of them. From the Valley of Spirits, run on the ledge past the Cleft of Shadows. You will them reach Grommash Hold. Not many places empty of guards are known.
Raiding Thunder Bluff
Thunder Bluff is the capital of the Tauren. Thunder Bluff is the second most populated Horde capital. Thunder Bluff is somewhat unique, as the entire city is located on four high mesas. Don't fall down, you will fall to a nasty death. There are two ways inside, two pairs of elevators. The first pair is located on the end of the road to Thunder Bluff. When you come up you will be directly on the main Square where the Horde players here will do their business. Run straight across the square and into the large totem pole tower in the center of the mesa. Run up the tower and into the first exit. On the other side of this square is the tent of Cairne Bloodhoof. The other way in is the other pair of elevators, located on the other side of the center mesa. Go up the elevator and into the tower again. Go out on the square where Cairne has his tent. Beware of druids when in Thunder Bluff. If they have Typhoon then can knock you off, sending you to a long fall.
Undercity is the capital of the Forsaken. Undercity is a bit underpopulated, but not by much. It is located under the Ruins of Lordaeron. The Ruins themselves contain no guards, but players will most likely run past. There are two ways down. One is the main entrance, which is a couple of elevators beneath the Throne Room. These will take you to the Trade Quarter. A lot of guards and NPCs are located here. Run down to the lowest level of the Quarter and get to the Apothecarium. In the middle is the way to the Royal Quarter. The way down is long and there are two elite guards next to every opening. When you finally get down, you will see Sylvanas Windrunner and (possibly) Varimathras. The other way in is the sewers. The entrance is on the western mountainside in the Tirisfal Glades. Hordes will most likely fly over your heads, as the people flying to and from Undercity will go through the Sewers. A bit down you will find a couple of guards and a hole that leads straight into the Undercity. Jump down and you will find yourself in the War Quarter. Go south to the Apothecarium and down to Sylvanas.
Raiding Silvermoon City
Silvermoon is the capital of the Blood Elves. Silvermoon is a very easy raid, but also the largest Horde city. The city is somewhat remote and not many Hordes go there. There is only one way in, the main gates. You will find yourself in the Walk of Elders. Run up the western way to the Murder Row and then northwest to the Court of the Sun and the Sunfury Spire which contains Lor'themar Theron and Rommath. Horde players who use the Orb of Translocation in Undercity will appear behind the Blood Elf leaders. Have an eye fixed on the Orb.