Tower City Center seems like a most visited place for tourist but because of it's amazing shopping or dinning options.
If you are taking the RTA train, the lower level is the main hub where all the trains connect. And the complex also have an inside walkway that connects to The Quicken Loan Arena and The Horseshoe Casino.
I love the architectural details of the interior. Make sure you look up at the beautiful ceilings and enjoy the courtyard area in the Main Concourse.