The way I designed the stage and how it plays.
When starting the stage I only had a few vague ideas. I knew how I wanted it to start and end, and I had a few ideas for puzzles. So the first thing I did was make Samus's famous ship and lay things out for the start and end of the game. Soon after I made the final boss area and fight knowing this was a very memorable part of the original Metroid, so it was important to get it right. The famous escape was added after.
I then had to focus on the power up progression structure and how it would be implemented into the game world. Having to work within the confines of Super Mario Bros. made this a bit of a challenge, but I think I managed to succeed.
Players start the game out as small Mario with few abilities. Then, unable to jump where the arrows go, the player will fall into a pit. From there I included three paths, two of which are impossible to get past at this time. The third leads to a Chozo statue with the Samus Suit Mystery Mushroom.
Upon gaining the Samus Suit power up the player is forced to learn that you can break bricks while they have it. This also opens up one of the other paths that was blocked by bricks. If the player goes this way there are some enemies, but nothing too hostile since Samus can't shoot anything. Soon the player will encounter a wall of spikes at a seemingly dead end.
This dead end is the first puzzle. There is a pipe nearby that spawns red Koopa Troopas. The player will have to survey the environment and figure out that they have to kick the red shell to the left, run with it, watch as it bounces off a wall and onto spike covered terrain, and run back over the safe route in order for that shell to activate a Question Block.
Once that block is hit a moving vine comes up that players must climb. It moves back and forth and requires players to watch out for the spike walls. This climb is meant to mimic, in a way, Samus's wall jump ability from the Metroid series.
Then the player is met with their first boss fight against a Sledge Bro. (Kraid). After getting past him there is the next upgrade, the Kuribo’s Shoe. I designed it so the player quickly learns the shoe makes it safe to walk on spikes. This new “power” opens up the third path.
At the end of the third spike covered path is a highly placed warp pipe and most players will abandon their shoe to get to it, which is fine. Upon entering the pipe is a drop that leads two ways. To the left is a door, and to the right is a locked off area. The locked off area is what I call The Armory and it’s a completely optional segment. It's meant to entice players to explore the game world more as there are more secrets to uncover. The rest of the stage is a bit more straightforward from here.
The next puzzle players will encounter requires unblocking a Bill Blaster so a Bob-omb can travel across a line of conveyer belts. Once it hits the end the Bomb-omb will break open some bricks revealing a path to the second boss, Bowser Jr. in a Clown Car (Ridley).
There are some short segments after this with some more Metroid references, followed by the big final boss fight against Bowser, who is meant to resemble the Mother Brain fight from Metroid, and an escape towards the end of the level. There is more to the design though because I made sure to cram this stage full of so many Metroid references that you'll probably miss some with only one playthrough.