Just kidding. Honestly, I love Snape mentors Harry stories; they were my gateway fanfic drug, and I still rejoice to find a well-written and convincing one.
So, rejoicing, here! :) The voice of your Severus is well done, with a refreshing self-awareness and clarity of vision. I love the gradual development of his relationship with Harry, how he adopts him by slow degrees, how he is changed by the eagerness and intensity that are Harry's hallmark, whatever universe he inhabits. And I like your Albus better than the canon one, mostly, I think, because his machinations are, what, gentler? More obviously infused with real love, anyway, and less aimed at some outside good than at improving the lives of the people involved. And Harry is lovely, with his constant questions and his irrepressible interest in everything, his ability to win people over--even staid, repressed, self-isolated people--charming and not overdone.
The details of the setting and of the process of designing and building the bridge are nicely handled--enough for verisimilitude, but never weighing down the story.
The ending made me teary, but in a good way. Well done!