This is Kastelli, a tiny secluded, pebbled beach on the uninhabited island of Kato Koufonissi, part of the Small Cyclades in the Aegean Sea.

The best way to access it is through Pano Koufonissi (this one is inhabited, with plenty of accommodation options). You either hire a speedboat company (there are a few right on the main port) or ask the traditional fishing boats to get you there. You can ask Mavros G Boat tours company and mention my name (Stef Greece). The only way to access Kastelli is by sea, there is no road or trail that gets you there. The boat tour doesn't last long, since it's quite close to the port of Koufonissi. With the speedboat it should not take more than 5-10 mins. Also, while on the boat, you should definitely ask the skipper to take you to a few more nearby secluded beaches of Kato Koufonissi, as well as Glaronissi. It's an uninhabited islet right next to Kato Koufonissi.

Bear in mind that throughout the months of June, July, August and September, the dominant weather in the Small Cyclades is dry, strong northern winds and clear skies. It's possible that the rough seas disrupt sea transportation, so always plan accordingly.

Also, if you wish to reach the beach, the best way is to get close with the boat and then jump in the water and swim to the shore. The pebbles are big and hurt your feet, so make sure to bring your sandals with you to the shore. Oh and..no littering!