The first step is to complete a computed-based test in their office to assess the problem solving capacity. I think it's a common procedure for any vacancy in BCG. It's not possible to use calculator or any electronic device apart from the computer where you are doing the case study. The test in timed and you only have 45 minutes so you fell under pressure while doing more or less complicated maths.

Multiple rounds with consulting case interview portions and fit portions. Each round of interviews had multiple interviews, and the interviewer gets more senior as you progress through the interview process