Belem tower, staircases done right

We've visited castles and forts throughout Europe, climbing what must be thousands of precariously narrow, spiral stone staircases. But here at the Belem Tower was the first time I saw technology used to enhance the experience--sensors indicate if others are on the staircase and determine their direction, providing those waiting with a green or red arrow in each direction to avoid awkward on-staircase human collisions. Brilliant.