The Martells are one of the most interesting houses across Westeros. House Martell began as petty lords of a small portion of land near the eastern coast of Dorne. This was until Nymeria, the warrior queen of the Rhoyne, sailed her 10,000 ships across the Narrow Sea and landed on the Martell lands. She took Lord Mors Martell as her husband and burned her ships so her men would have no where else to go. Together they conquered all of Dorne and ruled for centuries.
House Martell made Dorne very different from most of Westeros. Lands, titles, and crowns passed down to the eldest child, regardless of them being sons and daughters.
Dorne was never successfully conquered by Targaryen forces. Seeing the results of fighting on the field, The Field of Fire, and hiding inside castle walls, Harrenhall, the Martells ordered their troops to use guerrilla tactics to harass the Targaryen armies out of their lands. This worked for over 200 years until House Targaryen and House Martell joined together through marriage and brought Dorne into the empire that finally became the Seven Kingdoms. Their sigil is a red sun pierced by a golden spear on a field of red. Their words are Unbowed, Unbent, Unbroken.