D&D Tomb of Horrors TPK

Total Party Kill
It took three months, but the party finally met their death halfway through the heroic tier of the Tomb.
How did it happen? We broke our adventuring rules:
1. Never split the party: I faced a losing battle with a large-sized Undead while the others tried to take out medium zombies. In the back of the party the halfing rogue was putting distance between himself and the fray while taking potshots. A large Ivy monster took advantage of the stray halfing and grabbed him. At this point I felt that a retreat could very likely possibility, so I rushed over to release the rogue from the Ivy in preparation for the retreat. That only meant that I was now tangled up with the Ivy monster. The paladin replaced my stand with the giant, and the healer stood between us trying to keep us alive.
2. If a retreat is called, you must follow: Several times we said the word 'retreat,' but for some reason or another, we just kept fighting. There's really no excuse for this.
3. Don't do anything risky: I tried to save the halfling. We all at one point decided to risk our lives and not retreat.

So now that we're all dead and trying to figure out whether to resurrect or roll up new characters, I'm creating a few edits, and vow to enforce the rules more actively.
1. Never split the party.
   a) Focus fire on one target [unless there are multiple ranged enemies]
2. Follow retreat.
   b) Don't hesitate to retreat even if it's just outside the entrance to regroup and try again.
   a) If needed, leave other party members for dead, otherwise you're breaking rule #3.

Oh yeah, there was a 1 square hallway we could've just walked over to, where we wouldn't have had to fight the large creatures.