a) Look cool
Getting point a) takes a bit of creativity with stuff found in thrift stores and cheap stuff outlets like the local Action store here. Getting a few bits with interesting looking textures goes a long way if you want to transform polystyrene and dirt into miniature terrain.In preparation I always scout around the internet to make my own personal 'mood board' for new terrain. Just google a general term for whatever you're trying to build and pull a number of nice pictures of the internet for reference. For my upcoming project the search will be 'vampire castle'. Yeah searching is not a very difficult activity.
Making playable terrain is often overlooked by enthusiast terrain builders. For instance this manufactorum I build a few years ago looks (imho) quite nice.
But aside from giving a bit of LOS cover on the table it is mostly a big hindrance. It is in the way when you want to measure things and it is too high to climb during the battle (it would take a unit about 3 turns to get to the top. In the end if it is used it stands on the side just as eye candy.
Now when I design terrain I always think of making it playable first and foremost by asking a few simple question.
Can a unit cross it in battle?
1) can a unit conceivably cross or ascend it during a battle? For Warhammer 40K and Age of Sigmar average movement is 6" (give or take with some running) and a battle lasts between 5 and 7 turns. So any terrain that is higher than 12" takes more than 2 turns to ascend or cross, or about half the battle. If my terrain is higher or larger than 12" it should be an integral part of the battle, or I have to make it smaller.
How many models fit on or in it?
2) Can models fit on the terrain? For Old Style Warhammer Fantasy I used a 10 models x 4 model movement tray (20cm x 8 cm) for any terrain that should hold an average unit. For Age of Sigmar or 40K I want a small piece of terrain to fit at least 5 models on a 32mm base. Else it should be made in such way that models wont fit on it (and even then people can still find ways to fit their models on it anyway :s
Do I want to block LOS?
3) Do I want to block LOS (Line of Sight)? If so I will have to make sure it is really blocked (to avoid discussion during games). For instance if you're going to block sight with boarded up windows then make sure it is completely boarded up and one of the players wont be able to see a mm of a guy and still claim LOS.
What will the terrain do?
4) What can I do with a building during the game? I divide my terrain globally between hills, forests and cliffs. A hill elevates a units position, a forrest gives cover and slows movement a cliff blocks LOS and is impassible. Now to expand lets assume were building a house. This can serve as a hill if we make the roof flat and provide access with ladders or rubble. It can serve as a forest if we just build a few walls (for a ruin) or a ruined second story (for a 'forrested hill'). Last it can serve as a cliff for instance if I build an intact building with a peeked roof. Of course you can combine all three functions for instance in a partially ruined mansion. But it pays to think about what you will build in these game terms and plan for it.
So enough general rambling. Next up, planning the actual castle.