Castle Loch is the home to the Castle Loch nature Reserve. It is an SSSI with all kinds of wildlife to be seen on and off the water. in 2013, the Cvastle Loch Community Trust was set up in oder to prevent over commercialisation of the loch and safeguard the loch for the benefit of the community and nature. A walkway around the edge of the loch makes a very pleasant walk for an hour or two.
In order to find this cache, the nearby adventure lab should be completed. The co-ordinates are given on the final journal. You nmay need to download the adventure lab app if you haven't done so already.
Good luck!