The German joined City from Stuttgart in 2009 but never made a first-team appearance for the club.
"I was playing in the U16 national team at the time, and there were a lot of scouts at the games. At 16 you could sign your first professional contract. I had offers from all sorts of clubs back then, but Manchester City went out of their way to help me," Karius told Transfermarkt.
"They showed me a concrete plan, for example how I could make it to the senior game. That fascinated me. I could just as well imagine staying in Stuttgart - but the concept and their plan for me did not convince me as much as that of Man City.
"So I said to myself: I'm going to take the challenge and I'm going to England.
"Who else was interested? It felt like half of the Premier League. In Germany, I actually only dealt with Stuttgart because VfB was and is known to do great youth work. I enjoyed great training there.
"At that time I only listened to the plans of Man City and VfB. At that time, the arrival of investors [in clubs] was only just getting started.
"Man City was actually a club that relied heavily on its own youngsters. Many made the breakthrough, the academy was in a good position - that's why I decided in the end on this club."