From the CoE 3.26 executable:

Download[www.dropbox.com] the Conquest of Elysium 3 complete definitions of all terrain types in the standard game.

For the curious among you: the corresponding CoE4 data in the CoE4 forum.
________________________________________

Note, however, that many terrain properties appear to be hardcoded. They aren’t found in the terrain data block of the executable.

Perhaps most notable among these is the gem income of different terrain types. The gem income seems to be set and stored per individual map tile, only, during setup, doesn’t occur in the general terrain type data. I have nevertheless gone through the effort of manually testing what values to expect, and those findings of mine are also in comments in the c3m file.
________________________________________

Like before, the format is c3m files (plain text files) with pseudo-mod code. And, for the best reading experience, I recommend a text editor with CoE syntax highlighting, such as the schemes I have made for for Gedit and Notepad++ – or, if nothing else, the syntax highlighting of just a Unix shell script. (The last one will at least make comments and string literals stand out from the rest.)

Whatever you do, I would advise you don't use the Notepad that comes with Windows. (If you're on Windows, you'll be much better off with Notepad++[notepad-plus-plus.org].) Microsoft Notepad and CoE modding (or just about any serious text processing) are two things that simply don't belong in the same sentence.
________________________________________

Other data:

• Class and recruitment data.
  
• Weapon, spell and item data.
  
• Monster/unit data.
  
  
• Also, a spreadsheet with the data for terrain, items, weapons and spells in a more player-friendly format, plus, for modders, lists of sounds, fxIDs and monster flags.