I have completed the game but still haven't unlocked the codex entry for the Aztec Empire, even though I have already been to Tenochtitlan many times. Does anyone know where I must actually go to unlock it? It is the last entry to unlock and I just want to complete the collection :)

Thanks in advance.

That's strange, as I'm pretty sure I unlocked that without realising it - so I'm afraid I can't tell you exactly what unlocked it, although visiting Tenochtitlan sounds about right.

If you just want to unlock it so you have the whole set, and you don't mind cheating to do so, you can edit the Codices.ini file located in your Expeditions Conquistador user folder (located in Documents and Settings/<Username>/My Documents/My Games on WinXP, obviously slightly different on Win7/Win8).

Then just open it and add the following line to it:

Codex=Aztec

Then save it and restart the game and it should be there.

I actually found out how to unlock it properly in game after seeing your reply, but thanks for your reply nonetheless.