Hexadecimal editing means using a dedicated software to look at the files as texts written in a language which is based on the 256 combinations to encode a byte. The alphabet is made of 16 letters, 0-9 and A-F. Each byte is so described by two letters, from 00 to FF (16x16=256 combinations).
I had little time to check around the web but followed recommendations from a German site and downloaded WinHex and XV132, the first one allowing to compare files, the second to modify and save them.
Then the work is mostly similar to Civilization’s: you can deal with the MAP files which remain most often constant in size, or the SAV files which size changes every time you make any change. Working on the MAP files is easy, but does not allow much as most options you can set can also be done through the editor itself. Working on the SAV files should allow you nearly anything that does not make the game crash, but it is a challenge. The point is then for a game editor to edit a SAV file, make once for all some changes and upload –say- a game where you have a Ra temple complex but Ra is not in town, or where you have six floodplain farms of peas but peas are then disabled. One of my wishes was to set a map with 5 different temple complexes. Tricky, at least at my current stage.
I can also manipulate the text files but it is dangerous for players who are not used to save and move files around their hard drive. With this method, you submit a game that, at worst, crashes, at best is unique as impossible to arrange using the Cleo editor.
I am so busy in work that I had little time to investigate thoroughly and required tons of printouts to compare data. It is also much easier when you know what you are looking for.

Examples:
Events: I used the NAFTA map to find more about the earthquake: I removed all events except the earthquake. I found the event in A6D9A and could discover some other ones not allowed by the editor. The earthquake is the event 03. Its starting location is in 99FD6. The mummies of Civilis are 1D. I also found “a change of Pharaoh” at 05 and a “Riot” at 04. I did not bother about the change in the dynasty but was interested in the riot. I tried to make it happen playing but saw nothing: perhaps it is disabled, or I did not set the right parameters, or I did not set the game conditions that allow a riot to occur, it might even be in another city. All other events are allowed by the editor.
I did not find another possibility in the city status’ change than the ones known.

Enemies: looking for threads, I noticed a prehistoric dated one from Homegrown who mentioned that we could play with say the Romans. I had never noticed this one in now 3 years of gaming! In one MAP file, I change 9A031 to play 01with-00against the Hyksos. I then played at 99C70 with the 01 Sea People, 02 Hittites, 03 Mitanni, etc. I tried to play the Mitanni against the Hyksos but failed. It seems the first byte is a Yes/No option only. Can I be invaded by zookeepers? No, anything beyond a certain figure is Persian.

Predators: instead of lions or crocs, I managed to get in 99F08 scorpios in a normal climate area (03), lions in a humid place at 03, hippos far away from the water (settings snakes and changing the predator), hyenas at 04, scorpios at 05. I do not see how I could make it for game which also seems to be a Yes/No option.

Monuments and buildings: changing a 01 for a 1E I managed to have the right to build a step pyramid together with a caesareum, and altogether a mastaba that allow me to play with 4 different guilds; but I cannot get the Luxor Temple as it is disabled. I could neither see a bullfight pit. I tried to reenable the Temple Complex after I had set one but failed for now.

Items: In my upload for Buhen, you can see in a SY a stone that does not exist in the game, so I looked for it (however useless it might happen to be in the game). I offered myself anything from 01 grain to 30 Olive oil. Throughout is 15 (this famous stone), 32 is Leopard skin, 34 is Perfume, 25 is troops. I did not manage to receive “800 loads of troops”! I also see the image of the leopard skins offered but cannot get Leopard skins in my SYs as they are not available for trade. I have to look for it in making changes in the World map as it has to be available somewhere in another city for the SYs to mention it. As it is not a raw material, if I set it in my city the game simply ignores it. I could identify all the PICs known but not the one for leopard skins or the funny stone as trade items. Two holes in the list seems to show they were disabled.

Other features: whatever the changes you make, the bytes 99C3C to 99C43 change, even when simply opening the file and saving it under another name. Meaning?