At the heart of this groundbreaking fantasy series lies the battle for the Iron Throne, a prize for which almost every Westeros ruler seems prepared to die for. From the Battle of Blackwater Bay to the Battle of the Bastards, every major conflict in Game of Thrones has had some influence over who retains power over the Seven Kingdoms, whether large or small.
One of the most important success factors in every battle is the fortress being held and the effectiveness of its defenses. Let's take a look at the most difficult fortresses to penetrate in Westeros, some of which remain impenetrable to this day... despite that no longer being the case for the show's plot.
Riverrun
The seat of House Tully for generations, Riverrun is known for its high walls and its moat, which restricts entry to its narrow entryway. Petyr Baelish was fostered there during his childhood and remains in awe of its high walls.
This fortress was successfully defended by Robb Stark and the Tully's against a powerful Lannister army, and after the Freys took control of it after the Red Wedding, Brynden Tully retook it due to his knowledge of its entrances and passages. Clearly, this fortress is ideal for those with intricate personal knowledge of its architecture.
Sunspear
The ancestral home of Dorne's ruling house Martell, Sunspear has stood for over 1000 years and is yet to fall into enemy hands. In the center of this fortress is Spear Tower, a structure that is over 150 feet tall and is made entirely of shining steel. Despite the fall of House Martell, Sunspear remains the greatest fortress in Dorne, with each ruling house selecting it as their base from which to rule.
Highgarden
This ancient castle fortress was founded by Garth Greenhand, one of the kings of the First Men, thus making it one of the oldest castles around. It is located at the top of a steep hill, fortified by high, thick walls. Furthermore, as mentioned in the books, the outer wall and inner castle are separated by the "Briar Maze", which will surely slow down the activities of any invading army, thus buying the local army time to prepare. It was only captured one time in its history when it was successfully conquered by Daenerys' Unsullied forces, who laid siege to Casterly Rock; Jaime Lannister wittingly allowed them to do so while taking the majority of his army to topple House Tyrell for good.
Harrenhal
One of the oldest fortresses in the Seven Kingdoms, it also houses the largest castle on the continent of Westeros. So large is the inner fortress, that the grounds surrounding it cannot produce enough food to support the full army needed to man it. Originally, this castle was only conquered with the use of Aegon Targaryen's dragons, however, since that time control only shifted by means of death of natural causes or abandonment. For example, when Robb Stark's Northern Army seized it after the Lannister Army's withdrawal.
Storm's End
A Baratheon stronghold for generations, Storm's End is a coastal fortress surrounded by water. Its wall is 80 feet thick while there is a 150-foot drop into the water from its tower. It is also believed that there are spells woven into its walls that prevent penetration from magic. Throughout its history, Storm's End has never fallen to an attacker. Even during the Tyrell siege of Robert's Rebellion, Stannis Baratheon and his men managed to defend their fortress for over a year.
Dragonstone
This fortress is isolated from the mainland by Blackwater Bay, making it difficult and time-consuming enough to deter enemies from traveling there let alone launch an invasion. In addition, the castle itself is physically durable due to being built using advanced Valyrian stonemason techniques that have since been lost to humanity. Dragonstone was a reliable stronghold for both the Targayens and Stannis Baratheon, and due to its architecture, can be successfully defended by a small army against a much larger invading force.
Casterly Rock
The ancestral home to House Lannister situated on the western coast of Westeros, Casterly Rock is approximately an incredible 6000 years old. So well-protected is this fortress with its high walls, solid structure, and numerous floors that the Lannisters were able to protect it since Lann the Clever, the first Lannister, first acquired it during the Age of Heroes.
The only successful siege of the castle came at the hands of the Unsullied, which was only successful due to Tyrion's knowledge of the castle walls and passageways as well as the fact that most of the Lannister army was away at Highgarden with Jaime Lannister.
Winterfell
Only besieged once in its history at the hands of Theon Greyjoy and temporarily held by House Bolton, the Stark's historical stronghold was and continues to be a reliable fortress. It was said to have been constructed by Bran the Builder and consists of medieval-style buildings of stone that have stood for over 8000 years.
Apart from being difficult to conquer physically, Winterfell is also equipped to deal with the cold with many hot springs and fireplaces installed throughout the fortress. This ensures that members of House Stark are able to outlast any potential invaders during the harsh winter months.
The Red Keep
The home to every ruler of the Seven Kingdoms and their families, this fortress is located within Kings Landing, which has its own obstacles, defenses, and kingsguard that invaders would have to overcome in order to even reach The Red Keep in the first place. Daenerys' sacking of the city with the use of her dragon was the first time that it had been conquered in the true sense of the word, and it is not guaranteed that such a feat would have been carried out without the use of a dragon.
The Eyrie
Surrounded by the great mountains and narrow passageways of the Vale, the path to the Eyrie is enough to deter potential invaders. To make things more difficult, the pathway to the castle is constantly surveyed by guards who are quick to pounce on those who they consider a threat, not to mention that the passageway requires troops to move in single file. According to legend, this fortress is over 6000 years old, and House Arryn is yet to be unseated from what is surely the most easily-defended fortress in the realm.