Agree that is a documented problem - I haven't encountered it myself, largely because I don't let the AI siege and assault my cities

you have a couple of options in that situation - you can try to lure the reinforcements closer to your walls so you can engage them with towers and missile troops on the walls, or you can sit tight, go make a cup of tea and wait for the timer to run down and win the battle by default

not ideal I know, but it's not a game-breaker for me..

i don't have a problem with the byzantines trying to expand into north africa - ok it's a-historical (like much of the game) but it adds to the unpredictability of the game, and it's not unreasonable if you consider that (without the sea) they "border" you on that front