Her Majesty's Royal Palace and Fortress of the Tower of London: How aptly the title states the major roles of this iconic structure. Rising next to the River Thames, it was once the largest nonecclesiastical building in England and a focus of power for each new monarch. Over the centuries, the structure has been renovated, enlarged, and enhanced. As the architecture has changed, so has the Tower’s purpose. It’s been a fortress, a prison, and a palace.