The overall art theme of the game is loosely based on real world locations, theatres of war and the armies of real military powers. The WW1 and WW2 era feature heavily as the machinery, uniforms and locations have a wealth of interesting stylistic detail.
We used the strongest visual elements from these real world locations, armies, vehicles and additional stylistic influences (graphic novels, games, toys etc) using them as a foundation to build a new art style.
For example: WW1 battlefields + Military bunkers + Gothic architecture + Nazi Germany + Transylvania = Xylvania!
US Army + Sports + Action toys = Western Frontier
The environments also share a common influence, WAR! The architecture off all the environments is chunky and bunker like with slits for weapons and buildings that have been reduced to ruins by war.
Blending styles like this to form new location obviously takes a lot of thought and research however not all the environments contain so many stylistic influences.
The Corral Atolls location is pretty heavily influenced by the Pacific Islands with architecture based on native Polynesian huts and in order to add a stylistic twist we mixed the architecture and materials like bamboo and grass and with military style buttresses and gun slits. Reinforcing the base of the buildings with stone while the upper sections remained wooden

Note in the above image, the Japanese influence in the building shape, curved roof lines like Pagodas. The native village style materials like grass/straw, wood and stone. The military influence, bunker like windows with gun slits, the edges of the stone work are broken away by gun fire to indicate history, many years of war have passed here.
Fine details such as rivets, wood grain, brick mortar etc are all over sized to create a cartooney look and keep the surface of the models free from noisy, fine details that are associated with realism. There is also a simplistic and fun element to the designs which makes a lot of the vehicles almost toy like, chunky, bouncy with a feeling of indestructibility.