There is an item called Outpost Warp which mentions that higher level plains allow for more wildernesses to be controlled. The customer help says the following:
You must control a Plain of an equal or higher level in addition to the one your Outpost already occupies. If you transport your Outpost to a higher level Plain you will be given the ability to own more Wildernesses.
— KABAM Customer Help - Dragons of Atlantis: Outpost Warp
So, I went looking further using some terms, and encountered this guide to building a second city:
Be a Level 7 (Baronet): Do other quests first.
Have 10 Friends who play Kingdoms of Camelot: Google+ games are all about social.
Get a Second City Deed: Get 10 friends, finish the quest or buy the item with money.
Conquer a Plain: Choose a wise location (far away, so it survives), level doesn't matter.
The page also notes other useful information so you should read it, such as:
Not only will a second city allow you to conquer additional wildernesses, it allows you to raise buildings and armies simultaneously with your first city. Building a second city is a critical component to gameplay. By having one built, it allows you to double your resource production, double your army, and double your might!