I've unlocked all three locks on the cellar door but nothing happened!

Something did happen.

You just couldn't see it from where you were standing.

Go out and look around for changes.

There should be a new message on the BULLETIN BOARD, for one.

Gordon LAIR? Where might that be?

Do you remember having seen an as-yet locked door anywhere? Any time?

The door you've unlocked is not in this time period.

Step through the portal two times to travel back in time.

Walk up the steps to the Ruins.

The locked FORTRESS door is now ajar.