There are many useful spheres that you can craft for your units such as Gilded Pearl, Muramasa and many others. These spheres will greatly enhance your survivability, damage or your ability to resist status effects such as poison or stun from enemies.
To craft these spheres, you will need to upgrade your Sphere facility in town so be sure to read the Upgrading Your Town article.
To craft a sphere, follow the steps below:
Sphere Forging Proper
1. First you need to tap Town below.
2. After opening your Town, tap Spheres.
3. Select a sphere you want to craft and tap it.
4. If you have the materials needed to craft it, tap the blue Create button at the bottom.
Note that some spheres require other crafted spheres to be made so be sure to familiarize yourself with the list.
To learn how to equip your spheres, please go read the Equipping Spheres article.