If a defending player plays Shen during combat, can they target Shen with Fortified Position's 'when you defend' battlefield effect?

Ruling: No, Fortified Position's "when you defend" effect only triggers once when the player is designated as the Defender at the start of combat. Playing Shen mid-combat does not cause the battlefield effect to trigger again.

Sequence:

• When combat begins and you are designated as Defender, "when you defend" triggers go onto the initial chain
• You must choose a target for Fortified Position's effect at that time
• If you play Shen later during combat, the battlefield effect does not trigger again

Nuances:

• The battlefield says "when YOU defend" which refers to the player being designated as Defender, which only happens once per combat
• This is different from unit "when I attack/defend" triggers, which currently do trigger when units are added mid-combat (like Leona)
• You could target Shen with the battlefield effect if you play him before the showdown begins (such as during a pre-combat chain triggered by a move effect)