Please I am very dumb- wut do the crystals around towers mean?

The six crystals that vanish one after another? That's the Demolish rune, and when an enemy has it in tower range those crystals will appear. When every crystal has counted down the enemy can attack the tower for a large damage boost based on 30% of their maximum health.

EDIT: Guess I got it backward, the crystals fill up instead of vanishing.

Most likely referring to the effect shown when someone with the demolish rune is nearby. Once they all fill up, the demolish rune user can hit the tower for a chunk of bonus damage.