Castel Tealdo or Thealto was a former medieval castle forming part of the defenses of Ferrara.
The castle stood along the river, west of the town, as did the Castel Novo, and was built in the 10th century by Tealdo da Canossa, Count of Modena and son of Otto I, Holy Roman Emperor.
In 1317, the city rebelled against the rule of Robert, King of Naples, and locals overpowered the Catalan mercenaries who had taken shelter in the castle.
[1] This led to the admission of the Este to Ferrara.
In 1597-1608, this castle and the churches of San Giovanni Battista and Sant'Agata were razed to build a papal fortress.