Ávila Price: €4million
For six months of the year, Isla Centinela is surrounded by water in the Burguillo Lake. The rest of the time it is a peninsula and can be accessed by car.
It is located not far from Madrid Airport, and sits in the middle of Valle Iruelas Nature Reserve.
On the island is a Twenties castle and buildings which include a huge banqueting hall and terraces. It is currently used as a hotel with room for 34 guests. Agent: Vladi Private Islands (0049 40 33 89 89; vladi-private-islands.de)