The Training Arena was situated in the heart of Dragon's Edge. Designed by Hiccup, it had a retractable dome which could cover the length of the arena. It served as the place where the Dragon Riders trained new dragons and could also take shelter.
The Training Arena is designed with a structure created primarily from wooden beams. These beams support a foldable canvas dome that can be controlled to fall down either side, making an enclosed space within the arena, by using levers. This canvas is often installed in order to protect the arena from storms, or during dragon races, where the winner is the racer who gets back before the closing of the canvas. It is stated that this structure was designed and created by Hiccup, which would make sense, as he designs most of the mechanical gear and buildings for the Gang.
The wooden beams supprting the canvas are adorned in the shape of dragon heads, similar to many other wooden carvings within viking architecture.
The interior of the Training Arena is fairly bland, given that it is used solely as a meeting, fighting and training place, so no furniture or other such aquisitions are required. Occasioanlly, training dummies and other such equipment is set up for target practice in the arena, but these items seem to be taken in from other places, and are not permenantly situated within the arena. This lack of interior furniture would make sense, as the arena is not usually used as a place of comfort for anyone.
The Riders used the training arena primarily to train their dragons and practice maneouvers. Alternatively, they practiced their own fighting drills; this was mostly demonstrated by Hiccup and Astrid, who enjoyed sparring there on several occasions.
It is used as a place for the Riders to talk and meet in various episodes. It is generally where they discuss strategy, and is even used as a place to clean their dragons.
Due to its canvases on either side that can be enclosed to create a dome, the Riders and their dragons use it as shelter from storms/threats on several occasions, including the Fireworm Migration the Shellfire attack, and various catapult barrages.
The dome was designed by Hiccup and built by the Riders in "When Darkness Falls."
In Gone Gustav Gone, it was used as a training arena.
In Reign of Fireworms, it was used to shelter from the onslaught of fireworms.
In Total Nightmare, the Riders played "beat the dome," a game where they try to fly through the dome and outside it before it finishes closing. It is revealed that Snotlout has never succeeded at this. Snotlout manages to trap the titan wing Monstrous Nightmare in the dome, however, and uses the confined space to defeat it.
The dome is used to shelter from Dagur's catapults in Astrid's Team.
The dome is used many more times as a training arena and a shelter over the course of the series.