Okay, so when it comes to building I have a good sense of what i want to build and how it will look. But however when it comes to building a castle/fort I have no clue what I'm doing, so I'm calling out to all my Myner's throw some ideas towards me from internet images or your own ideas.
This is the area I am working with.
I have a philosophy of building that just says when you want to build, build. Lets the building grow organically with no plan. Sometimes I might end up tearing down whole sections if I decide it needs changing. But better to place the blocks rather than leave them to get griefed.
@MagnaOpera I do normally build organically with my builds but this is something I've not touched so I wouldn't now where to start interms of the shaping ect
building with natural stuff underground is extremely (sorry my vocab band is dead right now) contradictory? if done properly it can look extremely good. may i suggest something that may seem hard? build a glass dome around your garden then use ice to terraform the land, its not impossible it just needs alot of afking in a snowbiome. this way you will have your very own fully legit good looking underwater garden. kinda not whats being implied by ur area but i think itll look good. other wise a build like zabolcas would look good.
i would say build in the shape of the ravine and make a spindly narrow fort with segments of the walls the sides of the ravine
I honestly have no idea how you are going to build a castle in such small space, maybe you should pick an easier location like a mountain, island or maybe a hill in a forest.
or a bigger cavern
You can do it! Just take that first lip, mine back 5 or 6 blocks and boom little walkway for the village. At the end of the ravine add a full height (of the ravine) wall with windows and such. That's the main part of the castle. Then fill the bottom of the ravine with rare resource blocks or lava. Boom complete. Don't let these folks tell you what to do. Ravines rule.
It could even be a sort of tiered city, like artisans on top, builders in the middle, farmers on the bottom. Not that any of those 3 are lesser or more than the others, but I just couldn't think of better castes.
I think a castle tower would actually well fit in this space! I would suggest maybe building a stone outer frame and then having the tower wall one block behind it. Kinda like it has two layers almost, to avoid the old giant flat surface trap (I've fallen in it many times myself). You could also use the two layer idea to help break up the shape which would draw in the viewers eye. Also adding in little details like cracked stone bricks or just stone brick stairs inside the wall would help :)
I'll include some picks of one I just quickly made (so please don't judge). I only worked on two of the sides so ignore the flat one, but it's just so that there are pictures if my words fail me (just pretend it's underground) :P
(it's better if you imagine this as the ruins of an old civilization or something) 
I put a pile of cobble to look a bit like rubble or something, thought it would be fitting for an underground stone build and it also makes it look old like it's slowly crumbling. Idk gives it character :P
