Battle Arena
Season Number Available Details
Season 1 Chapter 4 Speak with Syrenne and Lowell near the Arena to gain access to the arena.
Season 2 Chapter 28 After clearing the infiltration of the castle, go to Ariela's Tavern and speak with Syrenne and Lowell. Choose Yes to go to the Arena and access Season 2.
Season 3 Epilogue This can be accessed during the Epilogue. As usual, Zael will find Syrenne and Lowell there. Go along with them to the Arena.

Speak with the three spectators that stand next to the Arena entrance to get free items/gold from them whenever you clear an Arena match.

Spectator rewards are: 750 G or Almond, Coconut, Wine or Gnome Copper x3