The fyrd actually predates Alfred and likely comes from the reign of Offa of Mercia in the eighth century, but Alfred is responsible for a major reform and expansion of the concept alongside his network of burhs. The fyrd operate on a tripartite rotation. At any point, 1/3 of the eligible men will be called up to serve garrison duty at their nearest burh or local stronghold. Another third would have been employed in carrying out maintenance on fortifications, roads, bridges and other important infrastructure, and the final third would have been at home, maintaining the agricultural functioning key to an agrarian economy.

The problem with the fyrd is that is it incredibly expensive to maintain, and Alfred has to go on a major PR campaign to convince his nobility that it really is worth it, although its success in dealing with the Vikings greatly helps him here. Works such as Asser's Vitæ Ælfredi contain cautionary tales about ealdormen who slacked their defensive responsibilities and were overrun by Vikings as a result. The relief for the system comes from the traditional campaigning season which starts in mid-Spring and ends in late Summer. This meant that as a rule, the fyrd could be released to go home to plant their fields and harvest their crops, since the enemy would also be at home doing the same thing.

The difference with the fyrd and other levies is the rotational system which makes them in essence a proto-standing army, and affords them a great deal of time to train. Anglo-Saxon shieldwall tactics require a great deal of discipline to work effectively, and the English were largely able to achieve that by having soldiers called up at all times rather than just in a crisis.