相关介质
连通现世、异界的井口、河流，或是开启海城墙内外大门的钥匙。人物关系：大洋居民已经有说到，这里再具体阐释一下。
即“二十年旋转周期”指的就是，现世（the World）与异界（the Outsider）各自地域先分别运行，然后产生的一个交点。这个交点，类似于两个行星在各自轨道运行，某天，它们相遇，打通了相互进入彼此轨道的桥梁。在现世地域中，桥梁的一头是海城墙内的某处“井口”；异界里，则是墙外的某条河流。
这是其一。简单讲就是：现世的井口、异界的河流。其二，海神渴望看望墙内出生的女儿希雅，特意打造出一把钥匙——能够连通两个不同世界，短暂开启海城墙的大门。
小说《云的国》作为《海城墙》的继续，存在连接另一个地方（云城）的介质，这里不方便透露。
结局
主要人物怛的死亡，薨抵达云层最后的藏身之所。该地却已经被另一个强大异界（云城）选做为通往人类海洋（现世、王国）的大门入口。（薨之烈火惨死？）新的战役即将展开。马修决定回到出生地大洋，找寻身世之谜，联合人类王城，共同抵抗新的强大敌人……
Media
The well mouth and river that connect the World and the Outsider, or the key that opens the gate inside and outside the Sea Wall. Characters Relationship: Ocean Residents has already mentioned, here explain in more detail.
That is, the “twenty-year rotation cycle” refers to the World and the Outsider each running separately, and then producing an intersection. This intersection, similar to two planets running on their own orbits, one day, they meet, open up a bridge to enter each other’s orbits. In the World region, one end of the bridge is a “well mouth” somewhere inside the Sea Wall; in the Outsider, it is a river outside the wall.
This is one. Simply put: the well mouth of the World, the river of the Outsider. Secondly, the Ocean God longed to see his daughter Sea who was born inside the wall, and specially made a key – that can connect two different worlds and briefly open the gate of the Sea Wall.
The novel “Cloud Country” as a continuation of “Sea Wall”, there is a medium that connects another place (Cloud City), which is not convenient to reveal here.
Ending
The main character Da’s death, Hong reached the last hiding place in the clouds. The place has been chosen by another powerful Outsider (Cloud City) as the entrance to the human ocean (the World, Kingdom). (Hong died in a fierce fire?) A new battle is about to begin. Matthew decided to return to his birthplace, the Big Ocean, to find out the mystery of his origin, and unite with the human King City to resist the new powerful enemy…