When the kings of House Gardener conquered the Reach, Lymond decided to marry into their family rather than fight them. By marrying a daughter of House Gardener, Lymond was able to transform the Hightowers from wealthy but relatively minor kings into the greatest lords of the Reach. Since then, House Gardener, and later on House Tyrell, has always defended Oldtown, allowing the Hightowers to focus on learning on trade.
In the books
In the A Song of Ice and Fire novels, Lymond is not mentioned. The World of Ice and Fire specifies that Lymond was known as the Sea Lion and fought and killed King Theon III Greyjoy after ceasing to pay tribute to the Ironborn king. He brought back the First Men tradition of thralldom long enough to make the captured ironborn work on strengthening the walls of Oldtown.