The approach has to be different from a lower level dungeon. After all, if gameplay stays the same, why level at all?

That means, to me, that you can't take away the PCs powers. No teleportation blockers, no indestructible walls, no giant antimagic fields. At least not everywhere. This goes double if we assume this is the final end of hte campaign and the PCs retire after this. You want to end on a high note, surely.

Now, the thing here is that your players may be immortal teleporting, time stopping, plane-altering demigods at level 20, or they may be monks and fighters. You can not make assumptions about what a "normal" group looks like on this level, because after a while, at least in third edition, "level" becomes an immaterial measurement of power. YOu can't make the level 20 dungeon, you have to make your level 20 dungeon.

What do I mean by that? Start by writing down your characters greatest strengths, and then challenge those. Your wizard likes to play five-dimensional contingency chess? Challenge them by someone who does the same. Your wizard wants to be the strongest there ever was? End a boss fight by wrestling a fire giant barbarian over the caldera of a volcano. Your druid reshapes the land with a wave of his hand? Drop him into Hades or Pandemonium or the Negative Energy Plane and make that the only thing keeping him alive.

And if your party consists of a sword and board fighter, a healing cleric, a sneaky rogue and a wizard who is really proud he just learned meteor swarm? Then you let them fight a stronger series of monsters in some hallways. That's perfectly alright too.

But tailor it to your party.