How would you go about making a multi-level tile based voxel terrain, similar to Starcraft 1/2?

Here is a picture from Starcraft 2 showing terrain at two levels, joined by a ramp.

How would I go about making a system similar to this terrain? Currently I have a 2d array of tiles, which i can apply different textures to. But this system is limited to only one flat level. How would I go about expanding this system, or redoing it properly, to achieve something similar to SC2?