I used to live in Kilsby and if I remember correctly, there are 3 of the 'castle' vents and 4 normal vents. The tunnel was the biggest civil engineering work to date at the time of construction. It was the first mainline railway tunnel over 2 miles long and the first civil engineering project to use steam power (to work the de - watering pumps). There was a massive camp for the navvies and many times soldiers from the garrison at Weedon (about 5 miles south down what is now the A5) were called in to restore order. They are stories, but no actual proof, that when drunk, the navvies would cahllenge each other to jump across the big 'castle' vents ....about 70 feet!! Robert Stephenson had a house in Kilsby and to this day there is a model of the portal of the tunnel in the front garden. Its about 3 feet high and 4 feet wide.