A retired doctor in America has built a replica of a Welsh castle, out of 50,000 Lego bricks.
Bob, from Illinois, took six weeks to complete the model of Rhuddlan Castle in Denbighshire.
And he showed how he did it by taking 2,500 pictures as he went along.
He is already looking at a new challenge: to make Caernarfon Castle which needs 60,000 bricks.
Watch Martin's report to see the model grow and grow.