Finely grate ½ teaspoon of zest from the orange and reserve for the salsa. Squeeze the juice from the orange into a bowl.
Pour the boiling water into a measuring jug and sprinkle in the jelly crystals. Stir to dissolve then add half the orange juice. Make up to 400ml (14 fl oz) with cold water. Pour into a mixing bowl and leave to cool until room temperature, stirring occasionally.
Gradually stir the yogurt into the jelly liquid then divide between four mini pudding basins or ramekins. Cover and chill for 2 hours.
About 15 minutes before serving the castles, make the salsa. Crush the sugar, mint and orange zest together then mix with the rest of the orange juice, the strawberries and peach or nectarine.
Carefully unmould the castles, dipping the containers into hot water very briefly if necessary. Serve with the fruity salsa spooned on top.