How do I cast an Orb of Presence?

[DnaielPartstone]

I'm at field F1 on Haunting Difficulty. When I hover my pointer over "Your Old Wizard Tower", I see "To access the battlefield, you cast an Orb of Presence." How do I do that and what happens when I do?

[strigvir]

The Orb of Presence is where enemies come to.

[DnaielPartstone]

Well, I know that. But how do I cast it?

[DnaielPartstone]

Oh, never mind. I get it; it's a narrative, not an instruction.