House Hightower is based in Oldtown in the Reach, with its seat in the great tower called the Hightower on Battle Isle. In the Game of Thrones TV series, they’re only mentioned briefly and never become a major on-screen faction, while in the books they’re much more important.

Where they are

Oldtown is in the southwest of Westeros, at the mouth of the Honeywine, and the Hightower rises there as both a castle and lighthouse. That makes the Hightowers a powerful Reach house tied to one of the oldest and most important cities in the realm.

In the show

In the TV adaptation, the house is mostly background lore. The most noticeable connection is Ser Gerold Hightower, the famous Lord Commander of the Kingsguard, who appears in the Tower of Joy context.

In the books

The books give House Hightower a much bigger presence, especially through Oldtown, House Tyrell politics, and references to Leyton Hightower and his family. So if someone says “where are the Hightowers?” in the show, the answer is basically: they’re in Oldtown, but mostly off-screen.

TL;DR

They’re in Oldtown at the Hightower, in the Reach, and the TV series barely uses them compared with the books.