It was a dome made up of entangled thorn trees.
Background[]
Briar Wood was the home of the Petalars. Lion-O and the other Thundercats arrived in Briar Wood in order to escape the army of Lizards led by Slithe, in the episode Song of the Petalars
The Thundercats aided the Petalars in finding the Cliff of Winds within Briar Wood.
Meanwhile, the Lizards used flamethrowers in order to set Briar Wood on fire, which heated the air providing an upward wind permitting the Petalars to fly out of Briar Wood.
After that, the Thundercats charged out of the Wood to attack the Lizards.