If you are not yet at the maximum level of cities for the amount of glory you have, you are in fact able to take somebody else's camp site and make it your own (YOU MAY NOT TAKE A CAMP OF THE SAME CIV AS YOU). To do so you must attack (not plunder) the camp repeatedly until its loyalty is reduced to 10 and then attack it one final time and destroy the Town centre... after doing so the camp will belong to the person who opened the attack (1st in) as long as that person meets requirements. Players will recieve a resource bonus in the mail for each hero involved in the final battle so make sure to fill up the attack slots for the takover to make the most of it.
Like the CZ battle up to 15 people from each side can pile in to attack/defend and 3 from each side (if it is defended) will fight at a time. Each attack on the city will reduce the loyalty by 10 and there is a 1 hour cool down between each attack so it can take some time to make it happen. All enemy troops inside the camp when you take it over are destroyed but you keep all buildings that were built by its previous owner. If you do happen to lose a camp site, you may build another, try to take yours back or steal somebody else's. Fighting inside camps is pretty straight forward at this point, towers are not active so no real need for them yet but walls can be quite handy (remember though that unlike buildings walls and towers do not respawn if destroyed). Gates are NOT functional yet, they are fused shut and do not open for either side. After an attack is launched on a camp there is a timer for friends to jump in and join you (15 person limit) and 3 attackers at a time may enter the battle in order of who was garrisoned first. Ignore the minimap completely as the dots do not line up with units/buildings actual location. Destroying the town centre gives the attacker a win, killing off all enemy units gives the defender a win, the timer running out results in a tie.
When building defensive structures keep in mind that defending units spawn at the NW and attackers spawn at the SE corners of the map. The battle map is larger than the city map so you can not box in spawn points with walls, anything you build can be encircled by attacking enemies so don't build a massive wall at the bottom of your city and expect it to be effective.
At this stage of the game it may actually be a good plan to build many layers of walls around the city centre to buy time to take out attackers, buildings drop very fast once under trebuchet attack so if an enemy siege unit reaches the town centre its pretty much all over.