source: Later, a bastion was built on the eastern approaches
target: A bastion on the eastern approaches were built later.
references: A bastion on the eastern approaches were built later. A bastion on the eastern approaches was built later. A fortress on the eastern paths was built afterwards. A bastion on the eastern approaches was built later. A bastion on the eastern approaches was built later. A fortification was built later to protect the eastern border lands. A bastion on the eastern approaches was built later.