The Great Seto Bridge linked the city in the Shinju-sama's barrier to the edge of the barrier where the Vertex lived. It used to be the battleground for the second generation of heroes, but was destroyed. It had not yet been rebuilt during the third generation of Heroes and it's doubtful it will be.
The family names of the first and second generation of heroes were written on memorial stones at the mouth of the bridge. Presumably, the names of the third generation of Heroes were also added.
Before the bridge's destruction, bells hung from the bridge's cables. The bells would chime only when a Vertex was detected and Forestize was about to begin, alerting the Hero's they were about to be called upon. After the bridge's destruction, the bells were destroyed and this was no longer possible.
The Flower Calming Ceremony Edit
Utilized by the second generation of Heroes, the Shinju-sama could activate this after the Heroes sufficiently damaged the Vertex. The bridge's cables would glow white and pink flower petals, most likely Sakura petals, would fall from the sky. The remains of the Vertex would then be banished beyond the barrier, defeated. After the bridge's destruction, this Ceremony could no longer be performed and Heroes had to seal and destroy a Vertex's soul on their own to defeat it.
Washio Sumi is a Hero Edit
During the second generation of Heroes, combat occurred on the bridge to centralize the damage and protect the city from being damaged. Washio Sumi and her fellow Heroes fight Vertex for the first time on the bridge's Jukai. They have been told they cannot allow the Vertex to leave the bridge and must stop it here. After it's sufficiently destroyed the bridge's Flower Calming Ceremony banishes the remains beyond the barrier.
They fought a second time on the bridge and also succeeded in stopping the Vertex and activating the Ceremony.
While on the bridge, the Heroes fight three Vertexes. Sumi and Sonoko are rendered unable to walk, so Gin attacks the vertexes on her own. She stops them from leaving the bridge at the cost of her life.
Sonoko and Sumi fight a Vertex here, the Vertex having interrupted Gin's funeral, and they stop it.
The girls get into a fight with three Vertex on the bridge, utilizing their new equipment and fairies. One of the Vertex, Leo, unleashes a huge fireball attack. Sonoko is able to protect Sumi, but is thrown out of Mankai herself. The fireball is so strong it obliterates the bridge, leaving nothing but an empty gap between the city and barrier where the bridge used to be.
In the real world, the bridge was left a ruined wreck, twisted back on itself, as shown above. Combat in the Jukai afterward took place around the city rather than having a designated combat zone, and the Flower Calming Ceremony could no longer be performed.
Yuki Yuna is a Hero Edit
With the bridge's destruction, combat in the Jukai now took place around the city, leading to more wide-spread destruction in the aftermath of battles.
Nogi Sonoko summoned Yuki Yuna and Togo Mimori to the bridge after they defeated a Vertex. She revealed herself to be a former hero and told them that when a Hero uses Mankai they undergo the Sange and permanently lose the use of a bodily function in exchange for Mankai's power. She also reveals to them that Heroes cannot die and are effectively sacrificed to Shinju.