In Italian, every time you want to say 'at the bottom of...' you use 'in fondo a...' It's just the way they use the word 'fondo.' It's the same with 'at the top of...' (in cima a).
Prepositions are weird in Italian, and often they won't translate over to English exactly. My best friend is Italian and his mum told me that with prepositions, if you're not sure, half the time you have to just guess and hope you're right. She struggled a lot with English prepositions when they first came to Australia, and even now after thirteen years she still uses the wrong preposition sometimes.