One of the first things we did when we started working on the Rome map was to research the different biomes/ecosystems covered by the map and then simplify/exaggerate these in order to create relatively unique looks for different regions. Specifically, since many aspects of the game were already divided into the four major faction groups (Romans, Gauls, Germans and Britons) we created relatively unique looks for these areas including distinct ground textures and tree types. Then, since Gaul is by far the largest area of the map, we further broke that down into Alpine, Lower-Alpine, Marsh, Ardennes and a default Gallic look. So, overall we've got roughly 7 or 8 distinct looks to the terrain which we're hoping will add some much needed visual variety as well as help players more intuitively identify where they are on the map. Some of these variations also cross over into the farm types as well. For example, while a generic grain crop is used across much of the map, we also include olive trees and applet trees in different places depending on the climate. We also have cattle farms as a separate resource building and we did at one point have vineyards in the game but decided to drop them for gameplay reasons.
In terms of the light levels in the game, we're aware things could often appear a little dark in the earlier games. This was largely due to a desire to create noticeably different lighting conditions to help designate the seasonal changes. However, in Rome we've now got far more noticeable season cues (leaves now change colour in the fall, fall off in the winter, bloom in the spring, etc) so we've reduced the range of lighting conditions so that it remains pretty bright for the majority of the time (see image below) and only darkens to a brief night effect to signify the end of the season. I have been working on some weather effects that do temporarily darken the game but I think the contrast adds some nice variety.