Save the Queen
Prior to rescuing Queen Leene from Manolia Cathedral, the bridge is out of service. The route to Zenan was blocked off to prevent further Mystic incursion. After the Queen is returned to Guardia Castle, a man guarding the broken bridge comments that a frog was seen swimming across the channel.
War against the Mystics
When Crono returns to the Middle Ages, Zenan Bridge was the primary route by which Magus' forces invaded Guardia. The Commander conducts the Guardia Knights to defend it. When the knights run out of rations, Crono returns to the Castle to tell Master of Kitchens, the brother of The Commander. Overwhelmed by the number of hungry mouths to feed, he refuses to send more rations, but as Crono leaves the Castle to relay the news to The Commander, the chef hands over Spiced Jerky for the troops, and a Strength Capsule as a reward for running the errand. When Crono returns to the bridge, Ozzie, with the help of his black magic, decimates the Guardian resistance. The Commander awards Crono with a Golden Helm for bringing rations and asks that he help keep the Mystics at bay. Intervening, Ozzie summons and Zombor to defeat Crono and his friends.
Before Frog opens Magic Cave with the Masamune, he has a flashback to an intimate conversation he had had with Cyrus, many years prior. Cyrus admits he will join the Castle Knights in their attempt to stay the Mystics. He urges Glenn, to join also. Frog discounts his ability as a swordsman. This flashback takes place at Zenan Bridge, overlooking the water below.
Zenan Bridge stands firm through the Present. However, on a map of 1999 A.D., the bridge disappeared. Likely it was dismantled due to the subterranean passages and above-ground sites connecting the dome structures of that period. The very same passages exist in 2300 A.D., verifying this.