While making levels in NSMBU / NSLU, it is important to be familiar with the usage rules and quirks of each tileset you will be using.
When you incorrectly use a tileset, be it a visual error (texture cut-offs, repetitive texture, etc) or a mechanical one (tile with no collision used as collision, or the opposite, etc), that is considered a Tiling Error.
Tiling Errors decrease the overall quality of your levels, so it is always good to check for them and fix any you find before considering a level final.
Below is a concise list of the most well-known Tiling Errors made by several modders in the past so that you can learn from their mistakes, aswell as detailed guides on how to correctly tile some of the more complicated tilesets in the game, such as Tower and Ice.
A Visual Tiling Error is a graphical error which causes the level to look visually displeasing, confusing, or straight up ugly. These should always be fixed when applicable.
Using the corner tiles is very important so that the levels don't look unfinished and generally ugly.
Here is another corner error, this time on the insides of the corners.
Very small holes in the ground simply look odd. Holes should be wide enough to be jumps, not too thin.
When using block-based tilesets, such as Rock-Candy Mines or Sparkling Waters, avoid blatant repetitive usage of the same block, mix and match different variants.
Try to make background rocks more diversified by adding the larger filler objects, instead of filling the area with only the randomized 1x1 filler object by itself.
Background rocks are pretty, but make sure to have the proper borders around them if they are exposed. Also try to avoid making overly flat borders.
When decorating your level with stone blocks sticking out of the ground, the background should be filled in with something such as background rocks, in order to prevent the outline of the stone block hole being see-through into the level's background, which usually looks weird.
Though it may seem obvious, some people don't know how to make multiple decorations on top of each other. Simply using different layers will fix this problem. Just be careful to not do this excessively, as too much overlapping decoration looks bad and confusing.
Pipes should always connect to solid ground. They can also be extended outside the zone to make it look like they connect to another part of the level.
When using Sparkling Waters tileset wood platforms, they should always be arranged so that both halves extend equal lengths, or two support logs instead of just one.
The mushroom stems should always be 1 tile inside the mushroom top. Make sure to use the correct mushroom stem sizes for the right mushroom top size.
The green pipe should go fully behind the ground. This is fixed by extending it further into the wall, then placing layer 0 tiles over where they intersect. If the tileset in use provides one, a cave wall is a good tile to use for this, as shown in the image. ---- The yellow pipe shouldn't simply go over the ground. This is fixed by carving out a hole for the pipe to go into, with background rocks behind it. The pipe also shouldn't abruptly end on a joint that leads nowhere, this is fixed by continuing the pipe downwards from the joint.
Mechanical Tiling Errors
A Mechanical Tiling Error is an error whose effects are not fully observable from within the level editor, as their issues are caused by in-game mechanics, such as how coins are collected or collision.
All tiles must be extended at least 1 tile outside of the zone border to ensure no visible cutoffs at the edges if the screen shakes. ---- When pits are added to a level, players die only after they fall beyond the zone border, thus, walls below the zone should be extended by 4 tiles. ---- Likewise, the top of the screen extends about 8 tiles above the zone border, making above-the-zone secrets possible, in places where those secrets are not intended, walls above the zone should be extended by 8 tiles.
Avoid placing platforms too close to the ceiling and next to a sloped wall, as the player might be able to glitch into the ground by crunching themselves into the slope and getting stuck, which will also be a softlock in uncleared levels.
step 1. cry
step 2. repeat step 1
step 3. give up
Overworld ice tiles have a sharp snow top, underground ice tiles don't.
Make sure the shades of blue match up all the way to the top / bottom. If possible, try to avoid making the pattern repetitive.
As can be seen in the image, all of these objects have completely solid collision, so only use them in places the player can't collide with.
There are 1, 2, 3, 4 and 6 tile wide ice objects. Make sure they are always at exactly their maximum width, or they won't tile properly.
Slopes have the same pattern as their flat equivalent, make sure to use them accordingly.
Because the ice tiles are transparent, background rocks should be extended far enough so they are outside of the zone boundaries.