If opponent has Frozen Fortress and forgot to trigger the Frozen Fortress and suddenly remembered, how does judge resolve this?

It depends on when they remember — the key is whether a new game action has been taken since the trigger's observable impact point.

If a new game action has been taken (e.g., they moved to scoring, drew, played a spell/unit): the trigger is forgotten (TR 506.3.b). It never goes on the chain, so the damage is not applied retroactively (TR 506.4; FAQ #11413).

If they remember before any new game action after the Beginning Step: it's not yet forgotten — the controller can still acknowledge it and put the Frozen Fortress trigger on the chain normally.

When a judge is called on a missed trigger (TR 702.2.f):

• Same turn the trigger was missed → the next opponent in turn order chooses whether to resolve the damage immediately or leave it missed (TR 702.2.f.2; FAQ #5809).
• Later turn → play continues; the damage is skipped (TR 702.2.f.1).

Penalty-wise: missing a mandatory trigger is normally No Penalty, but if missing it was advantageous to the controller (e.g., it would only have damaged your units, not theirs), it's a Warning (TR 702.2.e). Frozen Fortress's damage is sourced from the battlefield, so the accountable player is whoever controls it (TR 506.1.a).