This enigmatic structure is not Bruce Castle itself, which is a manorial piece of architecture. It is in the grounds and has its own Grade I listing separate from that of the manor house.
This tower is something of a folly - and not a Victorian one, it dates from the 16th century. To quote Nickolaus Pevsner:
"A great puzzle. [...] probably of early C16 date and of no known purpose. Its origins were already obscure by c. 1700, when Lord Coleraine wrote that 'in respect of its great antiquity more than conveniency, I keep the old brick tower in good repair, although I am not able to discover the founder thereof.'"
And we continue that tradition...