I was contacted online by a recruiter, whom I spoke with twice by phone. Firstly, I was sent a PDF with exercises relevant to the job, including web debugging, JavaScript and general website questions. You have to turn this in within a certain period of time, and then it is reviewed for approval.
Then, after my answers were approved, I had two sets of three-people interviews on-site, followed by a call with a manager, all over the period of approximately two weeks. So in total, I had 7 interviews. Some of them were on-site but with people via VC.
The interview questions varied from brain teasers, to behavioral questions, to Criteo's business model questions, to JavaScript questions. In essence, they ask about the things you need to understand to be able to work at Criteo.