The innovative game, Comanche, uses a technique called Voxel Space which is based on the same ideas like ray casting. Hence the Voxel Space engine is a 2.5D engine, it doesn't have all the levels of freedom that a regular 3D engine offers. wikipedia 
Sebastian Macke shows us this terrain rendering algorithm in less than 20 lines of code. github demo
The easiest way to represent a terrain is through a height map and color map. For the game Comanche a 1024 * 1024 one byte height map and a 1024 * 1024 one byte color map is used which you can download on this site. These maps are periodic.
Such maps limit the terrain to "one height per position on the map" - Complex geometries such as buildings or trees are not possible to represent. However, a great advantage of the colormap is, that it already contains the shading and shadows. The Voxel Space engine just takes the color and doesn't have to compute illumination during the render process.