I've been playing this game for atleast 2 years now, and even I havent been able to keep a fort descently through atleast two or three sieges yet. This of course is mainly caused by my forts being focused on 99% non-military stuff, as well as a lack of sieges due to forts being abandoned after a couple of years.
However the few rare sieges I have been through, have all been exciting and hilariously fail ^^
Only recently did I even attempt something like these:
-Huge intricate mechanism that does... anything (Failed before I got started)
-Huge megastructure, which would mainly be a box around an elven settlement (Failed due to slowness of water, but will probably be revisited)
-Creating an army larger than 5 dwarves. (Failed, due to lack of priority, again)
-Actually get an iron/steel/something-that-is-metal industry going on a larger scale (Failed due to GCS-trap-malfunction, followed by random siege, followed by random fun)
-Create a zoo containing only goblins, giant cave spider, or other non-normal creatures (succeded!)
Basically, what I'm trying to say is, you don't really need to do something huge, elaborate, or epicly hilarious to have fun in this game. I've had plenty of fun, even with my first few 2d-forts, despite their lack of anything-special-ness. Huge, elaborate or epicly hilarious happenings don't create a well-played game, they only create great material for stories, the other cool result of this game's existance.