1. Start with a new map 50 x 50. You will need a map this size to fit the moat, walls and manor. Then fill the map with green grass tiles. The green grass tile next to the thick / tall grass tiles.
2. Second, start by making a defense perimeter, like a moat, a pit, a lava stream or something like that.
3. Third, make the walls of your castle. Make sure there is room the walk between the moat and the walls.
--- Here is a good question to ask yourself, does your castle need walls? Do you want people to get up on your walls for another area to explore?
--- Do not cut holes in the wall for your path, you will do this later.
--- Remember there are height differences if you are making a second level of wall that people can walk on.
4. Now make the manor, which is the castle's keep. This is Where your king or land baron live.
--- Be creative, make it at least two floors and make it so you can walk around on the exterior of the second floor.
--- Be sure to use a different wall tile for the manor just to make things look different.
5. You need guard towers. Either classic towers that you can enter or watch towers that you need to climb up ladders to get to.
6. Make other buildings and sights to see. Make an armory, make the servant quarters, make a corral for horses, a hedge maze, an archary range or a grand courtyard.
--- They do not always need to be inside of the walls or easy to find.
7. Take this time to add paths that lead from each building to a focal point or the center courtyard. Now is the time to cut holes in your walls to put the path through. Make sure to add draw bridges and stairs or ladders to get up to your towers and walls.
8. Now spice up your ground textures with things like sand, dirt, tall grass and any other tiles you need. Take this time to add the floors to the second level of your manor. Make sure to add floor tiles to your walkways as well.
9. Add details like flowers, windows, doorways, bushes, trees, shrubs, ladders for your watch towers, signs and so on.
10. Put the final details with events. Make fountains, characters, guards, animals, butterflies, smoke from chimneys and so on.