Well, when speaking of realism, the reason why attacks were usually done at gate, by catapults flinging over walls or maybe by climbing towers is that even a fat oaken gate is a defensive weakspot compared to the several meters thick walls that can still be walked on to this day - and each df tile is like a dozen cubic meters.
When speaking of DF fort defences and your conception of them, though...Just in case you don't know, single-level overhangs are jumpable* (stronger creatures can jump greater heights as well I think?), walls can be dodged through or on top of, if your sieges are pathing to your main gate they're likely going towards something past that, thus meaning your courtyard has omnidirectional flier access^ and lastly of course all bridges can now be melted by dragonfire "battering ram" :p.
For what does the future hold, check the main site's dev goals. I don't recall it having artillery strikes and bunker busters, but it shouldn't be too hard to mod in a creature that fires cave in missiles if you want :p
I'm particularly looking forward to sending out sieges, myself.
"Just one guy, mayor?""Just one guy, outpost liason."*out walks the fortress champion Cog Datanguz wearing the Mires of Soothing, bulging artifact adamantine plate*That said, I'm not sure it is good idea to make building castles harder, given they're largely ornamental compared to their benefits - on the flip side, one can find unusual challenges fun - like no-roof outdoor farming when the weather is husking.
Sure, can just build a roof, but playing with one hand tied behind your back is rather common. Many players already avoid calm island embarks with double-airlock entrance to underground¯ or using bridges.
Digging invaders is pretty ancient idea (numerous mods incorporate them if you want).
That said, while I did find figuring out a trap design to defeat digging invisible tantruming omnidirectional clown car an interesting problem when I first encountered it (magma is love, magma is life)...
Having to implement one in every fort afterwards to be safe is differing only in scale in tedium to implementing smaller but same safe system several times. Also, it kills variety, organic expansions and interest if there's just 1 routine solution.
Perhaps you might have better RP with sieges being actual classic sieges if you limit yourself and following overseers in succession fort to only using aboveground free-range wild animals as food stock, though? I'm not sure what the food income from them is, but it might mean that letting siege sit around for multiple years like a proper invasion would run the risk of starvation.
^ I imagine if medieval invaders had flying dragon invaders we wouldn't have castles as design
* There is a story somewhere in ...what's going on in your fort thread, I think? of zombies spawning in wagon-only entrance and some getting past them to the fort.
¯ Not completely safe in vanilla game, to be fair, as vjek can attest from his ruin. Should you not deal with siege promptly, they will kill caravans and you will get unremovable ghosts, potentially killing your dwarves.
I put overhangs into my defences long prior to their being necessary (because it felt like I ought to) and I am now looking at my remaining "unnecessary, but feels right" tweaks, wondering if any of those will end up as necessary.
Oh yes, I was suddenly very glad to have already been planning for invisible clown maybe-thieves maybe-buildingdestroyers when I learned about invisible weremammoths.