Here it is, a fort I am making that will be hard for mobs, or players, to get into. Not completed at the moment, but I intend to finish it in due course. Its layers of defense so far:

Simple fence
fire
cobwebs over lava
wall with fence
'minefield' TNT with sand atop that drops into lava.
cobwebs
fence to prevent climbers
obsidian and stone brick wall with gravel inside, lava on top to fall down when dug. More obsidian and stone brick.
Not any more, I revised it. New:
-Two block tall nether fence with cobwebs on top. 'watchtowers' containing snow golems on all four corners, and in the middle of each wall. *Distraction*
-powered rail with constantly rotating minecarfts, whisks up attackers. And the rough villagers within distract beast valiantly, i would hope.
-Second two-block netherbrick fence. No watchtowers. Cobwebs on top.
-Classic lava trap. Cobwebs over two block trench with lava at bottom
-Iron doors, to stymie pig engineers and their silly ladders.
- Stone brick wall. Nether brick fence on top, and cobwebs partway up.
-Lava drop minefield of DOOM! (Awesome)
-Three block thick obsidian and stone brick walls. Center filled with gravel, with lava on top of the gravel to fall upon those who dig through.


Single entrance protected by:
multiple lava drops
nether rack flooring (To light)
dispensers
2 thick piston door
Not any more, it was proclaiming in neon; "Weak point-Attack here!" So I removed it. You will only be able to get in by enderpearl.