Siege of Storm's End

The Siege of Storm's End was a year-long siege during Robert's Rebellion. Following his defeat at the Battle of Ashford, Robert Baratheon fled north to Stoney Sept. Robert, then Lord of Storm's End had ordered his younger brother, Stannis, to hold the garrison at Storm's End and defend the family seat from the loyalist army.

While the rebellion raged on, Lord Mace Tyrell, Warden of the South, besieged the castle for an entire year. True to his orders, Stannis resisted and refused to yield despite being forced to eat the horses, cats, and dogs of the castle, and eventually even rats and glue.

Stannis and the garrison would've starved if not for the intervention of a smuggler named Davos, who managed to slip through the loyalist navy blockading Storm's End by sea and delivered a cargo of onions and salted fish. Thanks to Davos' intervention the garrison survived, until Ned Stark later arrived to lift the siege.

By the time Ned Stark arrived with his rebel army at Storm's End, Prince Rhaegar was already dead and his army smashed at the Battle of the Trident, and King Aerys Targaryen was already dead, with King's Landing sacked and in rebel hands. With the war clearly lost, Mace Tyrell gave no battle to Stark when his army arrived to lift the siege. Instead he peacefully dipped his banners and submitted to Robert's rule. The Tyrell force besieging Storm's End was the last Targaryen-loyalist army that had still been in the field.

Aftermath
As reward for his service, Davos was elevated to knighthood, becoming Davos of House Seaworth: he was given a castle and lands along Cape Wrath, and a place in Stannis' household for him and his children.

The Siege of Storm's End was of vital importance for the course of the entire war, because it tied down most of the large army of House Tyrell, preventing it from linking up with other royalist armies further north (from the Crownland, Dorne, and the Riverlands). If the castle had fallen, the army of House Tyrell would have joined Prince Rhaegar's army at the Trident, and overwhelmed Robert and Ned's rebel army. It was quite an impressive military accomplishment that Stannis held the castle against the Tyrell army numbering in the tens of thousands, with a garrison of only five hundred men. However, because it wasn't a dramatic battle but a prolonged siege (with occasional sorties), the siege and Stannis' accomplishment are not usually given much credit in songs and popular sentiment about the war. Stannis was left annoyed that his sacrifices and accomplishments were forgotten by so many, though Ned Stark himself never forgot Stannis' successful defense of the castle. Stannis also took it as a slight that after the war he was given the lordship of Dragonstone, while his younger brother Renly, a child who had not fought in the war, was given the lordship of Storm's End. Partially this was because Dragonstone was the ancestral possession of House Targaryen, with many staunch Targaryen loyalists, and required a firm and experienced lord to rein it in, and Dragonstone was also the traditional possession of the heir to the throne, but Stannis nonetheless felt that he had been snubbed.

In the books
In the A Song of Ice and Fire novels, the story of the Siege of Storm's End is the same, with Mace Tyrell leading the land forces and Lord Paxter Redwyne leading the loyalist fleet. During the siege, a few men of Stannis' garrison planned on opening the gates to the Tyrell army, which openly feasted before the walls of Storm's End. Stannis wanted to send them to the Tyrells by catapult, but Maester Cressen convinced Stannis to keep the would-be traitors imprisoned just in case the garrison was required to resort to cannibalism to survive. However the siege was lifted before such a measure was neccessary.