world
堺 centers on a bounded area or realm: a world, sphere, or domain. It often refers to a defined region or the world as a whole.
堺 combines 土 (earth) with 界 (boundary, world), reinforcing the idea of a bounded earthly realm. The exact historical development is uncertain, but the component structure suggests a semantic compound.
The earth radical 土 beside 界 (world) creates a picture of the physical, earthly world: 堺 is the world we stand on.
For カイ, imagine a kite flying over the world: kite -> カイ, and the kite sees the whole 堺 below.
Sakai school of tea ceremony