This is a paid asset, but now you can download CASTLE BATTLE ARENA ASSET PACK for FREE, Please keep in mind this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
CASTLE BATTLE ARENA ASSET PACK v1.0
Introducing the Castle Battle Arena Asset Pack that your players will love. Here are as many as 4 high-quality backgrounds (combat arenas) on which huge battles can take place for the honor of the king, or just to find out who is better prepared. All backgrounds are designed in 3840×2160 resolution. Thanks to this, you can easily implement it in most 2D game projects. RPG, action, adventure, arena battle, MMO-RPG and this is just a small part of the genres where you can use it.
THE ORIGINAL SET CONTAINS:
The ancient arena of the Coliseum;
Battle arena in the prison;
Cave arena;
Battle arena in the castle.
Integrate these backgrounds into your world, allowing players to enjoy battles, character leveling and unique locations. If you wish, you can easily add uniqueness to this pack. For example, you can integrate additional elements (chests, trees, torches, etc.), reduce the size, change the color scheme of roofs and flags, etc. To perform these tasks qualitatively, it is better to use any editor (for example, Adobe Photoshop).
Quantity: 4 Backgrounds;
Format: PNG, PSD.