The shape of Ingrew and surrounding area is something that I have thought long and hard about. The main religion in the Ingrewian Empire is that of Ansin (as of now, I plan to change this name later). There is an exact opposite religion known as Sekkan. Both these religions view the world as being a massive tree. The surface rule which contains Ingrew is nestled in a puddle in the upper branches while there are other realms in the lower branches. These lower realms are home to beings viewed as demons by the people living in Ingrew.
In reality I believe the world is shaped similarly to Earth except smaller and with a lot more water. There are only three continents and they have avoided finding each other for so long because of the vast distance between their shores. The whole great tree thing is more of an easy way to understand how this world works because there are other smaller realms, and the way Ingrewians view beings from these other realms is more representative of their fear of the unknown.