Ambria is a medium-sized planet in the outer-rim that has been ruled by the Empire since the fall of the first Republic. From the outside, it doesn't seem like a very usefull planet, hence the reason the NR never took control.From about 30°N to 30°S it's all desert. In that region there are cities, many of them. These cities are like mini mini coruscants, with tall structures, some, but few, reaching the clouds. North and south of the 30° parrallels there are forests until you reach the poles, which is ice.
The most important aspect of this planet is the secret Imperial construction and training taking place beneath several cities, underground. Until the Imperial takeover of the galaxy, no one knew about it, and even since the takeover, the only ones who do know of it, that didn't before, are the inhabitants of the planet, and they are loyal to the Empire.
Currently, the planet is as well protected as Coruscant.