Portugal
Castelo de Almourol is a small Knights Templar castle built on an island in the middle of the Tejo river. The only way to get to it is by small boat, ask at the cafe nearby if no-one's around. The boat will leave you for an hour or so to explore. It's remarkably intact for its age with battlements and towers to climb with viewpoints up and down the river, set in what resembles an overgrown garden.
www.igespar.pt/en/
Ilhota do Rio Tejo, Praia do Ribatejo - Vila Nova da Barquinha
+351 249 720 358
Google map: bit.ly/fRDDAx