Does Fortified Position's triggered ability go to the chain after when I attack/when I defend triggers, and when must the defending player decide which creature to have shield?

Ruling: Fortified Position is a defend trigger that goes onto the initial chain. The target unit must be declared when the trigger is put on the chain (when it finalizes onto the chain), not upon resolution.

Sequence:

• Fortified Position triggers when you defend
• The trigger goes onto the initial chain
• You choose the target unit as the trigger finalizes onto the chain
• Opponents can react with spells or abilities before it resolves
• Upon resolution, the chosen unit gains shield 2

Nuances:

• It is possible to kill the targeted unit with a spell before it gains the shield bonus
• The "choose a unit" wording indicates targeting, so the choice is made when the trigger goes on the chain, not at resolution
• This is not a reflexive trigger (which would require "do this" wording)