How is cheating possible?

[Evil Dave]

That being the case, there's no defense short of server checking, and there's a lot of checking to do. The type and quantity of every asset a player apparently holds (gems, items, units etc) in the incoming 2h file has to be reconciled against the ftherlnd file, and that's not easy.

I'm not sure it's so bad. I think all the server needs to remember is how much stuff each player started with at the beginning of a turn, and then get a list of new stuff each player's moves created. If a list cost more than that player had, something's gone wrong. Alchemy makes the problem a bit harder, but I don't think dramatically so.