Split the sponge cake and fill with butter icing, place in the centre of a cake board and brush with apricot jam. Cover with a layer of marzipan.
Cover each of the Swiss rolls with marzipan. Stick four of them around and cut the other four in half.
Tint two-thirds of the royal icing pale pink and cover the cake. Ice the extra pieces of Swiss roll and stick them around the top of the cake.
Use a cocktail stick to score the walls with a brick pattern. Make windows on the corner towers from jelly diamonds. Cut the ice cream cones to make the tower spires and stick them in place. Leave to dry overnight.
Tint half the remaining royal icing pale blue and cover the cones. Use a fork to pattern the icing. Shape the wafers for the gates, stick to the cake and cover with blue icing. Use the back of a knife to mark planks.
Tint the coconut with a few drops of green colouring. Spread the cake board with the remaining royal icing and sprinkle over the coconut. Stick on the marshmallows with a little royal icing to make the small turrets.