In my most recent playthrough, I noticed some disturbing and disjointed effects from the summoned units we call city defenders. Now, this has been posted the world over, but I might have some special insight given all the nuances my mod has added to the mechanic. And while, yes, this is a modded experience, I just checked it in the vanilla game with the same results. The problem lies in the summoning mechanic for city defenders. It works fine for the first battle, but after that, the summoning is not triggered.
What do we see on the second battle? We see a single unit most of the time with 0/X Hp. But this unit does not spawn. In fact, it seems the summoning of city defenders takes place before the battle is initiated, which is likely where the bug comes from. You see, most of the game makes checks and refreshes each turn. Because of this, the game may be resetting the summons only once per turn. However, once in a while, and often times on the third attack on a city we get our defenders back. So is it simply checking for defenders and making the wrong conclusion about whether or not they have been summoned yet? It could be that the game sees that they were there in the last battle and thinks they are still there. Or perhaps it sees a unit with 0/X Hp as still being alive and so no new unit can be summoned. But then there are several times when you won't get them back after the second time they spawn. It's very hard to make a conclusion without seeing the underlying code.
But even without seeing it, I can make some suggestions. One would be to add this bug to the list of things to fix for 1.03. We really don't have much of a game if sending one suicide unit to clear the city defenders is the best siege tactic. It's even worse that not trying to exploit this bug still exploits it. You either win with the first army or you definitely win with the second. The only other option would be to wait a turn to attack fairly and that is not fair to the attacker. So how can you fix it? If there's no easy solution, just make each city only attackable once per turn. A bit of triage so that we can at least say the game has no game-breaking bugs. I hate to say it, but this qualifies as one. After that you could put some more time into fixing the overall mechanic for the expansion or 1.1 depending on how much love sieges are going to see. It's something that is broken and needs some expansion. Seems like a good candidate for the expansion. Wouldn't you agree?