Battle Drill

Tower of Light: Opens on Monday, Friday, and Sunday. (You can use only Lightbearer heroes.)
Tower of Nature: Open on Wednesday, Saturday, and Sunday. (You can use only Wilder Heroes.)
Tower of Eternity: Opens on Thursday, Saturday, and Sunday. (You can only use Graveborn heroes.)
Tower of Will: Opens on Tuesday, Friday, and Sunday. (You can only use Mauler heroes.)