The tower, known as the Panopticon, allows Memnarch to keep track of all of the plane of Mirrodin. Note how in the art, Memnarch can see the Oxidda mountains, a shot of the Lumengrid, and other scenes. Note that the card Darksteel Citadel shows an exterior shot of Memnarch's Panopticon.
The word "panopticon" refers to a design for a type of prison by Jeremy Bentham in the 1700s. The design called for a radial layout of cells with the prison warden at the center, allowing the warden to see all the inmates at once. The principle of the panopticon was later used as a sociopolitical metaphor by philosopher Michel Foucault.
For more on the "Watcher" depicted on Eyes of the Watcher, check out this style guide shot of Memnarch. (The term for the mind-expanding liquid derived from blinkmoths, labeled as "lymph" here, later became "serum," as referenced in cards like Serum Visions and Serum Tank.)