The Tower Bridge Pack is part of the Castle Builder System. It is made of "Dwarvenite" and includes some metal and magnetized parts. This item was initially available as an add-on to a pledge level during the Castle Builder System Kickstarter. The item comes in both painted and unpainted versions.
The Tower Bridge Pack comes with these 3 pieces:
(2) Stone Bridge Soaring Arch
(1) Stone Bridge 4in Extension