The interview process is pretty thorough at Tough Mudder. If they like your cover letter and resume you'll be sent an invitation to a video interview. This is something I haven't seen before but it's pretty efficient. You log on and they have a list of pre-recorded questions for you to answer. You record your responses via webcam and submit the interview once it's complete.
Once you get through that stage, one of the Hiring Managers will request a skype interview, which is primarily intended to be a culture assessment. Culture fit is huge for them.
After the skype you'll have several rounds of in-person interviews with people relevant for the position you're going for, along with other people from the company who sit in. Like I said, they're pretty thorough. They want to make sure you're qualified from an experience and culture standpoint.
Overall, it's a long process but definitely worth it. Tough Mudder is an awesome place to work with really cool, smart employees and their interview process reflects how much they value their people.