How are people defending Odessa? It doesn't start with a garrison (let alone a hardened one) or an IDM. The only way you can get a hardened garrison there is if you play the Medium card that allows you to do it (sorry, forgot the card name, it's the one that allows you to convert 4 IDMs or garrisons to Fortified defenses or hardened garrisons respectively). The earliest and probably the only opportunity to do this is in the August I turn (you can't get a Medium card before then as the Soviets). That would seem to be the prescribed/mandatory play if you are to get anywhere near the historical result of having the city hold out for a 10 week siege. Without a hardened garrison, I don't see how it can hold out that long even with a maximum stack (14 defense) as any retreat result will kill the stack.