I realized End of Year 0 that without good water access and with 2 UW nations (R'lyeh north and Ys East) and C'tis as neighbours, I needed that chokepoint-province to break into the mainland to the West either by war with T'ien Ch'i or due to a cave entrance in that chokepoint-province.
I took the chokepoint and most of my peninsula (with 2 thrones) by Summer of Year 1, I also got a treelord staff during year 1 which helped my cast Mother oak without boosting or climbing.
T'ien and me had a Mexican fort standoff at the chokepoint, during which I gobbled up the cavelayer with 11 provinces and a throne, which gave me access to 3 thrones (5 to win).
Purposely, I did not claim it to not seem like the big threat since I already had the only global up. I don't know why T'ien did not expand into the caves because he also had access to it, but I had the lucky situation that I went into the cave FROM my border chokepoint, and he would have had to divert an Army from his side of the chokepoint to do so.

End of year 1, Ys took my trying to push their filthy kind back into the water (where I completely underestimated Morvarc'h knights and embarrassed myself) as aggression and attacked me with said OP sacreds.
Just in time I came to an agreement with T'ien who probably wasn't too happy himself parking his army at the chokepoint and we signed a NAP.
My glorious unicorn knights swiftly took care of the slimy villains and Ys went AI.