On the river.
Vimacopeca — CC BY-SA 4.0 · Allied Joint Force Command Brunssum — CC BY-SA 2.0 · Vimacopeca — CC BY-SA 4.0 · Threeohsix — CC BY-SA 4.0 · Threeohsix — CC BY-SA 4.0
Almourol Castle is a small stone fort with crenellated walls and towers on a rocky islet in the middle of the broad Tagus River, ringed by trees and water, so it looks like a castle from a fairy tale when mirrored in the river. It is thought to stand on the site of an early Lusitanian fort, later taken by the Romans and held by many peoples after them; Roman coins and foundations have been found here. In the 12th century it became part of the Knights Templar's line of river defences, and it is among Portugal's best-known medieval castles and clearest Templar landmarks. You take a small boat from the bank to the islet and climb among the rocks to the walls and the keep, with the river stretching away on both sides. It is about 4 km from Vila Nova da Barquinha and about 16 km in a straight line south of Tomar. The boat may stop when the river is high, so check before you go.
On the river.
And walls.
Short crossing.
Almourol islet, Tagus River, near Vila Nova da Barquinha
By small boat from the bank.
Yes, in one day.