It's possible that castle is where Robbie and I are currently located. The stonework I'm seeing inside matches the outside.

[It's the same color stone.]

That said, it would perhaps be unwise to knock or otherwise try to open the doors by force from the outside. There's no telling what kind of alarms might be in place and it would be for the best that the beings that Robbie saw frozen in place, er, stay frozen.

But there may be a way for us to safely open the door from within.