dwell (up)on someone or something
to remain on the [important] subject of someone or something for a long time. (Upon is formal and less commonly used than on.) I can't dwell upon this subject anymore. There is no need to dwell on Sarah further.
dwell (up)on something
to live on something, such as the planet Earth. (Upon is more formal than on.) This is the largest turtle that dwells upon the earth. Many creatures dwell on this earth.
*in an ivory tower
Fig. in a place, such as a university, where one can be aloof from the realities of living. (Typ—ically: be ~; dwell ~; live ~; work ~.) If you didn't spend so much time in your ivory tower, you'd know what people really think! Many professors are said to live in ivory towers. They don't know what the real world is like.