Heat the oven to 160C/140C fan/gas 3. Grease and base line a round 13cm cake tin and a round 18cm cake tin. Place two muffin cases in a muffin tin.
Put the butter and sugar into a large bowl and beat together until light and creamy. Add all the remaining ingredients except the jam and beat well. Half fill the muffin cases and then fill the two cake tins 3/4 full. Bake in the oven placing the cake tins on one shelf and the muffin tin on another. Open the oven door and remove the muffins and smaller cake quickly when they are done as you don't want to let the heat out of the oven. The muffins will take about 35 mins, the 13cm cake about 1 hour 10 mins and the 18cm cake about 1 1/2 hours. You know each is done when a skewer inserted into the centre comes out clean. Allow the cakes to become completely cold before icing.
Trim the tops of the two cakes so they are flat. Slice each one in half, spread with jam and then sandwich together again. Using a 6cm cutter, cut each muffin to make a cylinder. Trim the tops flat. Sandwich together with jam to make a tower.
To make the butter icing put the butter into a bowl and beat until soft. Gradually beat in the icing sugar. Spread the butter icing over both cakes and the muffin tower. Roll out the white icing and use to cover the two cakes. Place the larger one on a cake board. Put the smaller one on top. Roll a strip of white icing and wrap around the muffin tower. Place on top of the cake. Push a wooden skewer though the cakes to hold them together.
Using about 150g pink icing, mould into an onion shape for the dome. Put to one side. Roll out a little pink icing and cut out a 6cm round and place on top of the muffin tower. Using the made up royal icing, attach the dome to the top of the tower.
Using the pink icing make two doors and two curtains. Attach to the cake using royal icing. Make windows by rolling out the lilac icing and cutting out hearts using medium, small and tiny cutters. Attach these upside down to the cake with royal icing. Roll a small piece of lilac icing into a thin sausage and stick around the curtains to make the window frame.
Again using the royal icing, stick on the pieces of gum around the doors, and the sugar cubes around the edge of the cakes. Attach silver balls to the dome to decorate and to the doors to make handles. Score horizontal lines down the doors to give a textured effect. Attach a piece of gum under each window on the middle tier and then stick three flowers on top to make window boxes.
Colour the remaining royal icing green and spoon into a piping bag fitted with a plain nozzle. Pipe creeper branches up the castle walls. Attach mini sugar flowers.
Stick coloured flags or candles into the middle tier of the cake. Sprinkle a little edible glitter over the dome and sugar cubes.