I was contacted through LinkedIn to apply for this job. First, I had a phone interview with a recruiter that explained me everything. Then, an online challenge in hackerrank, which I did well. Finally, I was asked to travel do Madrid, where I was supposed do have 2-4 technical interviews. The first did not go well, it was probably my bar raiser. The other went ok. I guess that if the bar raiser has the power to reject an application then the first interview was enough to reject me. It is also worth mentioning that it is hard to tell what the interviewer is thinking, they give no feedback. On the overall it was a great experience and the tests are not that hard of you are well prepared. My advice is to spend a lot of time doing exercises before applying.