Tips: The  Rom

Distribution
I do not assert that the following is perfectly accurate and it is not intended to be used as legal advice, nor does it condone any specific actions or behavior. Ensuring that your actions are legal is solely your own responsibility. That being said, these guidelines should give you an idea as to a good way to distribute your modified ROM.

The NES game, Metroid, is a copyrighted work by Nintendo and is illegal to distribute, in part or in whole, to anybody who does not own an original and legally obtained copy of the game. (Due to recent litigation the simple act of making the copyrighted work available to those that do not legitimately own it may be considered illegal, even if the file is ultimately not copied illegally.) This does not mean that you can not distribute your own edited worlds. The changes made to the game are intellectual property created by you and you legally own the copyright to the new data that was added to the ROM. The recommended way to distribute a modified ROM is to use an IPS utility to create an IPS patch. The IPS utility examines an original copy of the ROM and your modified ROM and identifies differences in data, creating a patch that contains only the changes you've made, and excluding any original (copyrighted) material. The recipients of the patch then use an IPS utility, in conjunction with an original (presumably legally obtained) ROM, to reproduce your customized version of the game. (An analogy could be made between distributing an IPS patch and distributing a mod for a more current game: you don't distribute the game, only the mod.)

Before creating an IPS patch, it is also recommended that the ROM be saved without map data (unless you intend for other users to open the ROM in Editroid). Look under OptionsàMaps. If the "Save To ROM" option is selected, change it to "Save To External File." Open the ROM if it is not already open. Now, save them ROM. The map data will be excluded from the ROM and placed in an external file. Once you have obtained a copy of the ROM without map data you may set the option back to "Save To ROM" if you prefer.

Red Doors
One of the things you will probably notice early on is that when you add new red doors (the doors that have to be opened with missiles) the doors will not remain open like the pre-existing red doors. You need to update the game's password data (see Password Data Editor under Editors) so that the status of these doors will be stored in the password.

There are also other considerations with red doors. It is important to make both sides of a door the same color. If one side is red and the other is blue then the door will not work properly. You are also limited in the number of red doors you can place since there is only a limited amount of password data to work with (although you may find the behavior of a red door without associated password data to be acceptable).

Enemy Slots
Enemy slot assignment is important; poorly orchestrated enemy slot assignments will probably result in enemies not appearing where you want them when you want them. Certain enemies might not ever appear at all. The reason is while the game is being played, there are six "slots" (numbered from zero to five) allocated for enemy data. When enemies placed on the map are scrolled into view in the game, they aren't simply assigned to a free slot. Instead, the slot is defined statically and when the enemy is scrolled into view, if its slot is taken, the enemy simply won't appear.

There is no single rule that can be followed to ensure that all the enemies appear when you want them, but there are some general guidelines to help things work out more like you planned. One technique used in the original game is to repeat a single screen (or multiple screens with similar enemy setups) with a large number of enemies filling most slots. As the player goes through the screens the enemies may appear to be random since a number of factors are involved when it comes to which slots will already be taken. You may also want to designate a "reserved" enemy slot that will be used sparingly. When you want to guarantee that a specific enemy will appear, it can use this reserved slot that will usually be free.

Palettes
When editing background palettes you have a lot of freedom; do whatever looks nice. There are some things to keep in mind, though. You may notice that by default the first color in every palette is black. This color can be changed, but the first color should be the same for all palettes. This is necessary due to the technical implementation details of palettes on the NES. (For those who are curious, the NES "mirrors" the first color for each palette, meaning that the first palette entry is shared between all palettes. This means that when a palette is loaded into the NES video memory, the first color of that palette overwrites the first color of all other palettes, so all palettes will have the same first color as the most recently loaded palette, which can result in undesirable or unpredictable results.)

You also need to make considerations for palette swaps. For levels with two sets of palettes, each set should probably have at least one corresponding identical palette. The room where the palette swap occurs  should contain only objects that use the palette (or palettes) that is common to both sets. Otherwise the transition won't be seamless: colors in the room will suddenly change when the player walks through the door. A similar consideration is the palette used for elevators. The elevator screens should use the same palette number in all levels.

When editing sprite palettes, keep in mind that palettes are shared between doors, enemies, Samus, and the life/missile display, and that certain palettes will be updated and overwritten when the player switches to missiles, gets the varia suit, or becomes suitless Samus.