Applied through my university's career center. Had two back-to-back on campus interviews - one with someone from hr, and another with a trader. My first one was with the hr person, and it was just the standard collection of behavioral questions. Nothing too interesting. The second one with the trader surprised me by also being very behavioral. We basically just chatted about Citadel and fixed income trading in general. I thought it went really well, but I soon received an email saying that I would not advance to the next round.