Fortification enchantment on the stronghold's oversized door that turns force back on the attacker.
Force Reflection is the second of three enchantments listed when Carl touches the Oversized, Cumbersome Door and an info box appears: Apprentice Quality, 80% strength, 3/3 enchantments active — Battering Resistance, Force Reflection, and Heat Absorption — with the note that increasing quality increases enchantment capacity.
The box also carries a blunt warning: "You don't want to be behind this thing once it's kicked in." Wards of this sort are what the Stronghold's enchantment staff under Langrim are applying across the fortress before the Ceasefire ends.
One of three enchantments on the stronghold's main door, reflecting incoming force.