central — Something that is central is in the middle of a place or area.
interior — being within; inside of anything; internal; inner; further toward a center: the interior rooms of a house.
middle — equally distant from the extremes or outer limits; central: the middle point of a line; the middle singer in a trio.
certain — If you are certain about something, you firmly believe it is true and have no doubt about it. If you are not certain about something, you do not have definite knowledge about it.
definite — If something such as a decision or an arrangement is definite, it is firm and clear, and unlikely to be changed.