It was in two parts. The first was a casual Skype interview, the second was a day at HQ in Luxembourg. I got to stay at the "intern house" the night before in Luxembourg, which was a unique (if pretty full-on) way to start get to know Etix and the people in it.
The day at the office was loosely structured and casual. There was plenty of time between interviews to chat with people in the break room/kitchen (in fact it was encouraged), and everyone was friendly and open. The interviews often felt more like discussions or chats and were informative.
I interviewed first with HR, who had me take a personality test beforehand and based the entire first interview on it (which was the only part that I didn't gel well with), then spoke with two of the product managers, the head of HR (the "Chief Happiness Officer"), their current UX/UI designer and the front-end development team.
Overall, I got a happy, communal and often playful vibe from the company and the people in it (I saw some kind of crêpe party going on in the break room in the afternoon and finished the day with a foosball tournament and a beer with the front-end team). It was a really good day - I had a great interview experience overall and would have loved to join the company.