Does playing a damage spell like Disintegrate at an opponent's unit at Fortified Position start a showdown and trigger Fortified Position's 'when you defend here' ability?

Ruling: Playing a spell does not start a showdown or trigger "when you defend here" effects. Showdowns are only started by a unit of a player that does not currently control a battlefield moving into that battlefield.

Nuances:

• "Defending" only applies during combat when two players have units at the same battlefield
• Damage spells targeting units at a battlefield do not count as defending