I think that only the towers roofs are billboards. Everything else are just great texture, good lighting and models with few, well placed polygons.
I don´t think that they use any LOD, but instead they have choosen an environment which is perfect for some good occlusion culling. The view of the two last screenshots btw aren´t running completely fluent on my iPhone 3GS. There also aren´t that many different textures used, which is good to save draw calls. Well in the end they even seem to have enough speed left, which they seem to spend into antialiasing.