Sieges can lock down huge amounts of land, even without large unit numbers, for multiple turns, making it impassable for uninvolved bystanders and probably even affecting tile yields of uninvolved player territories (how do I even check if exploitation is affected?). If you have a unit inside that huge area, it will be imprisoned, unable to move at all, until the siege ends, which can be long, and it doesn't even matter if your unit stands on your own territory. Hover text over the "locked" symbol just says "This army is locked into a battle and cannot move until the battle has ended", and the siege battle window will tell you "You are not involved". 

If you are lucky, the battle ends at some point, but that doesn't always seem to be the case. During that time, you can't escape from inside that area, and you can't pass through from the outside either. Another threat in a different subforum: General \ MOST INFURIATING THING I'VE EXPERIENCED IN STRATEGY GAMES IN THE PAST 5 YEARS - LOCKED INTO BATTLE

The lack of options to interfere on behalf of the defender (an Independent People with me as #1 patron right outside my border in my case) was frustrating enough even though I understand that battles involving more than 2 players would be too complicated to implement, but I didn't even get any grievance for it either.

auto-save 120.ctr

The even weirder part happened afterwards though. (When the siege ended with the Independent city still independent, I guess that meant the defender won? I wouldn't know, I got no info about it, even though my locked unit was 2 tiles away from the city. )

I proceed to try to besiege the city myself that turn, in it fell to me without any battle. Not even a window where I could choose between manual or automatic, I just get the city. It should have been a siege battle between 117 me and 72 them, but no such thing happened. The only action between the second-to-last picture and the last picture was a right-click, the change in ownership was instant.

I'm aware of the possibility that the part with the huge completely locked down area for siege combat, spanning multiple areas and even involving third party units and (possibly) tile yields, might be intended. In which case it's just really bad design, and I can move that part to the other subforum.

The instant city capture most likely isn't though.