It's never occurred to me to build a line of forts to a landlocked city to give said city a passage to the sea. Wow! I usually don't build Forts because I don't use the Civic(s) that grants them Hammers or Gold.
It didn't occur to me either... that may actually be useful. However, the workers do it reflexively as a religious rite.
I mean that my workers are building a line of forts from a fort only junk tile (at least at that point in the game) directly to the fort they build directly outside my city.... they build an armored invasion route. Granted this is mostly from desert/ice cities unless marsh or something do it. The forts are composed of terrain entirely inside my culture borders and the Zone of control extends entirely within my borders. Meaning the sole purpose of these forts is giving invaders fortifications to invade by,.. I never order them to do it... its just the worker religion to build endless forts.
Maybe an AI close to you has Neanderthal Embassy and is making neander units to hem you in?
On turn 1-200ish in an eternity game? I haven't actually seen that building... but as they are spawning in the forest I have my doubts. Good move, for them, if there is a bulding that lets you control where the barbarian units spawn themselves. I tried an early raging barbarians game and it would actually speed up my growth (ignoring the lack of terrain improvement tiles) if I got

I know its likely being fixed... but if raging animals attacked my towns (and couldn't pillage) I'd enjoy raging barbarians and a way to speed up my games. The Moat of Doom, with those tantalizing animals randomly flowing through it, combined with all the forests spawning directly towards me like an organic tower walking attack... the Neanderthalls might as welll be the Zerg. My early culture's paranoia as seen in their mythology must be insanely Lovecraftian... like the Summerian mythos. The Gods are actively trying to kill them... only instead of with flash floods its with ominous creeping forests that spew forth axe weilding Doom!