The founding of the castle is said to date back to the 11th century.
Literal
Castle [possessive-の] founding [topic-は] 11th century [to-に] date-back [quotative-と] conveyed.
創建 'founding, original construction' of a building. さかのぼる 'to go back, to date back.' ~と伝えられる 'it is said/reported that' — passive of 伝える, creating a distanced, authoritative tone. Historical/encyclopedic register.