Warden is a title bestowed upon the head of a Great House by the King of Westeros. The Wardens command the armies of one of the constituent regions of the Seven Kingdoms in the name of Iron Throne for the defense of the entire realm. Thus any one Warden is expected to lawfully command a quarter of all armies in the realm during a time of crisis.
By custom, the title is functionally hereditary. However, a Warden does not necessarily have to also be one of the Lords Paramount (rulers of the constituent regions of the realm) though this is almost always the case. For example, if an elderly or infirm ruler came to be the head of House Lannister, the office of Warden might be delegated to another family member young and capable enough to lead armies in the field.
Current Wardens of the Seven Kingdoms
- Warden of the North: Currently held by Roose Bolton, Lord of the Dreadfort, for his part in the betrayal and murder of King Robb Stark during the Red Wedding, until Tyrion Lannister and Sansa Stark sire a child and he comes of age. The title was previously held by the head of House Stark, since the days of Torrhen Stark, the King Who Knelt.
- Warden of the West: Title held by the Lord Paramount of the Westerlands and Lord of Casterly Rock, the head of House Lannister, since Loren I Lannister, the last King of the Rock. Currently held by Lord Tywin Lannister.
- Warden of the East: One of the titles of the Lord Paramount of the Vale, historically the head of House Arryn, Lord of the Eyrie and Lord Paramount of the Vale. Currently held by Robin Arryn, the child Lord of the Eyrie.
- Warden of the South: Title first given to Harlen Tyrell, Steward to the last King of the Reach, after surrendering Highgarden and the Reach to Aegon the Conqueror, and traditionally held by the Lord of Highgarden and Lord Paramount of the Reach. Currently held by Lord Mace Tyrell.
In the books
Typically, the Warden of the North defends against Wildling attacks that come south of the Wall. The Warden of the West, from House Lannister, defends against coastal raids from the Iron Islands. House Greyjoy weren't chosen to be Wardens, because more often than not they are what the Warden of the West is defending against. Even though they submitted to the Iron Throne, illicit ironborn raiding would occur from time to time and have to be suppressed. The Warden of the South, from House Tyrell, would defend against border skirmishes with Dorne, as Dorne actually retained its independence for two centuries after the Targaryen Conquest, and was only united to the realm (by marriage alliance) a century before the War of the Five Kings. This is why House Martell of Dorne are not Wardens. The Warden of the East, from House Arryn, seems to have defended against raids or invasion from the Free Cities across the Narrow Sea, such as the threat during the War of the Ninepenny Kings when an invasion was mounted from the Stepstones in the Narrow Sea.
House Tully were not Wardens, probably because the Riverlands are centrally located within the realm and far from external threats. It is not clear why House Baratheon were not named as Wardens, as the Stormlands border Dorne to the south (and thus they could have been Wardens of the South), and also border the Narrow Sea like the Vale (and thus could have been Wardens of the East). The Tyrells may have been chosen as Wardens of the south due to the larger army and population of the Reach, and because more Dornish attacks may have been targeted at the fertile lands of the Reach instead of the forests of the Stormlands. House Arryn may have been chosen ahead of the Baratheons as Wardens of the East because they had to deal with contentious Hill tribes in their mountains and thus needed the military assets at their disposal to deal with them. Also, the Andal Invasion thousands of years ago began in the Vale of Arryn, so it has been used an invasion corridor across the Narrow Sea before.
After Jon Arryn died, Queen Cersei manage to convince King Robert to name her brother Jaime Lannister as the Warden of the East, because Jon's only son and heir "Sweetrobin" Arryn is just a child (his son's real name is "Robert" in the books, but this was changed to "Robin" in the TV series: either way he is more commonly known as "Sweetrobin"). This greatly angers many lords in the Vale because Jaime has no connection to House Arryn or the Vale, so they begin to defiantly refer to Sweetrobin as the "True Warden of the East". Jaime never actually traveled to the Vale or took command of its forces during these few months, thus his hold on the title was entirely nominal. Jaime continued to technically hold the position while he spent a full year as a prisoner of the Starks after the Battle of the Whispering Wood, though his imprisonment meant that the position was left functionally vacant. The title of Warden of the East is restored to Sweetrobin by Tywin Lannister as one of the conditions of the marriage-alliance between Lysa and Petyr Baelish which brings House Arryn back into the Lannister fold. The TV series omitted this subplot.