Tower Shop becomes available when the Tower unlocks at team level 15. You can find it by tapping Merchant in the bottom-left corner of the Tower screen:
You can also select the Tower tab when you open any other Shop:
Here, you can spend Tower Coins to purchase hero equipment, gold, and Soul Stones of Kai, Orion, Dante, Maya, and Fafnir.
New goods appear every 24 hours at 9:00 PM your in-game time. You can also refresh the Shop using emeralds.