I'm a novice DF player. My first fortress ended with lots of fun when I've overlooked a tiny bit of evil biome that resulted in wolverine severed undead head rampaging and killing most of my dwarves. The second one was met its demise when a pretty early goblin ambush has massacred my militia, killed half of my dwarves and made me ragequit.
I've therefore decided to take special care with my third fortress. I've found a nice, peaceful spot and started digging with security and military might in mind. A drawbridge was put in front of the entrance, dwarves were conscripted as soon as I could spare any while military industry was put on a high gear. The first goblin ambush was dealt with easily, second and third too. I've ventured into caverns extremely carefully, walling off entire sections, establishing barracks there and killing everything that could pose any threat. The first forgotten beast came and was quickly slaughtered with no losses on my side. The second one couldn't even find a way into my fortress and is currently swimming in a cavern pool and idling. The goblin siege ended with 20 invaders and just one poor axedwarf dead (The poor fella got his arm ripped off and then proceeded to wander around the battlefield with "Pickup equipment" job until he bled out. Kind of reminded me of Omaha Beach scene from Saving Private Ryan with a soldier picking up his own severed arm.).
All in all I felt that my focus on security was paying off and I could finally feel safe. The fortress was booming as I churned out luxurious clothes and furniture, set up new industries and kept my population happy and productive (after all, the greatest threat to security comes from own disgruntled citizens with too much time on their hands). My undead bookkeeper, sealed in his office, was doing a great job and I marvelled in my fortress wealth. I've actually started to think for a moment that things were getting a bit boring.
Then I became the Mountainhome. I knew that my queen was a vampire and thought I could contain her in her chambers with a raised bridge. "Eight thousand kills? That's pretty badass". While I pondered the use for such badass killer vampire dwarf queen I got a message:
"Urist McArchitect cancels Construct Building: Interrupted by cave crocodile head"
Turns out my vampire queen's husband is a necromancer. King consort necromancer. My whole civilisation for its entire history has been serving a vampire and a necromancer. And now my king consort necromancer has decided that my refuse stockpile outside my main entrance is a bit bland and needs more movement.
I guess I might handle this situation. I don't really know how to fight the undead so I'll have to learn it the hard way. If my king consort necromancer decides to give up raising dead, I should manage. If he keeps generating more zombie rotten Wild boar hair that strangle my dorfs, I'm in trouble.
Anyway, the reason I'm telling you this is to express my awe that even when you think you're safe and know this game, DF still has ways to throw lots of Fun in your way when you least expect it.
Thank you, Toady One. All hail Toady One.