turtle, tortoise
龜 centers on the shelled reptile: a turtle or tortoise. The two meanings are essentially the same animal, with tortoise often implying a land-dwelling variety.
龜 is a pictograph of a turtle seen from above, with the shell, head, legs, and tail stylized into the modern character. The form is ancient and well-established.
The top part suggests the turtle's head and front legs, the middle boxy section its shell, and the lower strokes its tail and back legs. Together, 龜 is a turtle.
For キ, imagine a turtle carrying a key on its shell: key -> キ, and the turtle slowly moves with the key.