Ueda Castle Park is the city's main sight and the reason the Sanada name still hangs over Ueda. Only gates, turrets, moats, and stonework remain, but the site matters because the Sanada twice held off Tokugawa forces here in the late 16th century.
The park is pleasant even if you are not deep into samurai history. Come in autumn for maples, or pair the walk with the small city museum inside the park.