Application consisted of four stages: an initial application, a video interview, an assessment centre and a final interview.
The video interview was as you might imagine, "why would you like to work here", "tell us about a time when..", etc. The questions aren't too difficult, but you aren't given a lot of time to answer so try to stay on-message. Also, showing your enthusiasm is really difficult during video interviews, but makes a big difference!
The assessment centre was genuinely really enjoyable. There was a series of group exercises, but these were not like horrible group discussions I've had at other assessment days. In the first one we were told that we were to image our plane had crash-landed in the jungle, and we had to choose from a set of items which we would take with us. In the second, we were given an egg and asked to construct a device from standard stationery that would allow us to drop the egg without breaking it. There was also a short written exercise and an interview, as well as a chance to meet recent graduates. Attending the AC means getting to Cambridge, but ARM were really fantastic about help with flights/hotels/transfer etc. Everyone I met during the day was really nice, including ARM employees and other applicants.
The final interview was conducted over skype. Again this was based around fairly standard questions, but specifically looking for me elaborate on the skills and experiences that were directly relevant to the position.