These are some facts about levels which I find interesting.

Daigoro 1 - Block Blow Go
  This is the only level in the entire Daigoro which you cannot
  save every lemming on.

Daigoro 12 - Road To Nowhere / Daigoro 13 - Road From Nowhere
  These two levels are almost exactly the same, the only difference
  being that the entrance and exit locations are swapped, and only
  Road To Nowhere has a repeat version. (Zanmato 17 - To The End!)

Kozuka 4 - Forgotten Dream
  The repeat version of this level is one of the two levels that
  were made for L+3 but were not included in the final set. The
  repeat version, "Into Eternity", was pulled from the position
  of Zanmato 16 due to being way too hard to pull off. (The other
  rejected level, "Desperation", was pulled from Wakizashi 14 due
  to having at least 3 unfixable backroutes)

Kozuka 11 - Fuck
  Possibly the only level I've ever made with a swear word for a
  level name. ^_^

Kozuka 18 - Downwards Bound / Zanmato 18 - Grounded
  Downwards Bound was originally exactly the same as Grounded.
  However, I thought this was too hard for a Kozuka level, so
  I copied the level to Zanmato 18 and renamed it, and made the
  original much easier.

Wakizashi 4 - Teh Dingo Pwnz j00!!11!1!11one!1!1shift+one!1!
  Umm... the name?

Wakizashi 6 - Level / Zanmato 12 - Zanmato 12
  Is there any other level called "Level" or named after it's
  position? Not that I've made. Not that I remember. Not that
  exists yet probably.

Wakizashi 19 - Copyright Infringement Part 1 /
Zanmato 4 - Copyright Infringement Part 2
  These two levels are equal for largest horizontal size in
  L+3 (1120 pixels). Part 2 is also the only level which gives
  a large amount of builders but requires them all.

Zanmato 2 - Survivors Will Be Shot Again
  This is the only level in L+3 that I haven't been able to
  acheive 100% on, but believe it's possible to do so on.

Zanmato 9 - The Way Across
  The only level in L+3 that uses a trick that could be defined
  as cheap.

Zanmato 11 - Down The Middle We Go
  As far as I know, the only ever Lemmings 3D level remade in 2D.
  Also the level that has the largest vertical size (800 pixels),
  and the largest overall area (256000 pixels).

Zanmato 15 - Warped Out
  The only L+3 level that was taken from a previous pack. (Well,
  technically not from a pack, as it was released as an individual
  level. And yeah, the Zanmato "Lane Of Lament" is almost identical
  to an Ultimate Challenge 3 level, but it has it's differences)

Zanmato 17 - To The End!
  The hardest level in L+3. Enough said.