A level must be composed of something, and therefore we have placed down this minimum number limit of eight tiles to ensure a level is always made of something.
We wouldn't want the player to be able to fly off into nothingness and get lost, would we? To ensure this doesn't happen, the level must be completely closed off with tiles
In order for a tile to be valid, it must be assigned a tile in accordance with it's tile neighbors and the level as a whole. If this doesn't happen, such as a tile being placed in an invalid/incorrect position, then this becomes an invalid tile.
Here is an example of a 'valid' tile and an 'invalid' tile:
A level must always have atleast one launch pad and one land pad. In the case of co-op levels, it may have two of each, but a level must never have less than one of each.
In order to be able to start the level, the player must be able to spawn from within the level.
In order to be able to complete the level, the land pads must be inside the level.