Tower Shop becomes available when the Tower unlocks at team level 15. You can access it by clicking the Shop tab in the bottom-right corner of the Tower:
You can also select the Tower Shop tab when you open any other Shop:
Here you can spend Tower Coins to purchase hero equipment and Soul Stones of Kai, Orion, Dante, and Lilith.
New goods appear every 24 hours at 9:00 PM your in-game timezone. Also, you can refresh the Shop using emeralds.