Yoshida Castle was the old centre of power in this part of Mikawa, guarding the Toyo River and the Tokaido corridor. The keep is gone, but the riverside stonework, moats, and reconstructed Kurogane turret still make a good first stop in Toyohashi. The park around it is broad and calm, with the city museum, sports grounds, and plenty of cherry trees.
Come by tram rather than taxi if you can. The short ride from Toyohashi Station puts the castle in context: this is a working modern city with a few older layers still visible between the municipal buildings and riverbank.