The Skirmish system will allow all players -- raiders, casuals, mid-level, high-level -- access to the same content. Twelve new Skirmish instances will be available in addition to a pair of tutorial instances designed to introduce players to the system. Each of these instances is scalable based on the number of players; they each accept one, three, six, or twelve players at a time, with the deficit made up for by player-created NPC soldiers- to make a grand total of 24.
Additionally, many of the instances are scalable by level. For instance, one skirmish which features an assault upon and defense of Weathertop will accept players from level 30 to 65. Each instance has nine optional challenges, each of which awards skirmish points -- more on those in a moment. These challenges are randomized, as are the enemies inside the instance (within certain logical limits, of course), so you will not likely have the same experience twice.