Yes, El Grillo is right that you cannot do this. The only way for him to move through the forts would be to destroy my culture on the tiles. Logan and Shiv's culture prevents this.
So, there's a few things Oxy can do here, but I'm struggling to see what would work which is why I'm slightly worried I'm missing something. But this could definitely be a desperate hail mary.
1) He could slam 18 Knights into Connor next turn. However, reinforcing the city enough to prevent this is trivial. In addition, while the cultural defenses are down to 0, it's still a city on a hill
behind a river. In general attacking Connor is weird because no matter what he's going to have to tank the river malus, and it doesn't really open me up quite as badly as Shiv would. Worth noting he does have more Galleons in range that
might be loaded to join amphibiously, so I will have to overstock it slightly. (Ignore the "9 galleon" sign, it's outdated, but there are a few in the area for sure.)
2) He could move one tile towards Connor onto the Fort. This lets him include his one-movers in an assault on Connor. This would be annoying in that it would make it harder to reinforce the city via boat. I could pre-fort the Horse though to help deal with this - then I could still ferry units in. It also blocks his ability to fork and gives me more time to reinforce.
3) This might just be wave 1 with more coming. Notably there's no siege anywhere in this stack. However, I know some of that is because in taking the island he lost siege and/or the siege is still healing. Going by power numbers, I don't think he's got
that much more in reserve.
4) He could try to run straight west towards the capital. The hills block his Knights from moving too quickly though, plus the cultural borders are untouched there, so that sounds like suicide.
5) He could pivot south towards Shiv. However, if Shiv is the target, his one-movers will take 3 more turns to be able to attack it which is 3 turns of adding Grens. Seems like a bad move for him. The Knights could get in range to hit it next turn, but again that pulls him away from the other cities.
If I had to guess he's going with some combo of 2 and 3. However, I think I have that all covered
depending on how big wave 2 is. If it's small, I should be able to hold here. The big issue for him is he's got no siege whatsoever here. I've got 7 catapults, a good amount of Muskets, and a quickly increasing number of Knights in the area. Plus I can start upgrading Zerks to Grens next turn. Vodka told me I can't quite attack his stack yet, but that was mostly due to lack of hitters, not due to inability to get odds.
He does have a GG that I have not seen which concerns me. However, worth remembering MilSci unlocks the Commando promotion, so he does not have access to it.