First off here's a some information about the game for those of you who haven't played the game. Paper Mario The Thousand Year Door (TTYD) is a turn based RPG released in 2004 by Nintendo illustrated in a colorful paper-themed art style. This was the second game in the already established Paper Mario series and is the sequel to the original "Paper Mario" released for the Nintendo 64 system to the world in 2001. It's most well known for its engaging plot, characters and creative gameplay.
There are three core systems that make up TTYD that I want to address which are movement, social systems and combat. Each of these core systems contains multiple secondary systems. These three main systems create a core loop that revolve in either direction as each system can possibly lead to another.
Firstly we have the core system of movement which contains the secondary systems of exploration and puzzles. Movement like many other games is essential in TTYD for the rest of the primary systems to function so this system always comes before the other two. In TTYD Mario has the unique ability to move around both 2D and 3D spaces depending on the situation. Being paper, Mario is given the ability to fold his body from a paper sailboat capable of moving across bodies of water to a paper airplane able to reach far distances. There are some abilities such as turning ninety degrees and facing away from the player that grant him access to difficult to access areas but fitting through extremely small spaces. The first secondary system being puzzles can greatly rely on movement and how the player will use these paper abilities to overcome different movement restrictions. Exploration is the next secondary system in movement. Many hidden events, people and information that may or may not be essential to the story can be found by simply exploring your surroundings. These events may have no indication of their existence and could be hidden in areas that the player may never necessarily need to go to. These are here to give more life to the environments in the world.
The social systems in TTYD are the second core system I'll be addressing. Talking to NPCs and gathering of currency are vital parts of the social systems in the game. Without these things combat would become very stale with little variation in available items, allies, health, etc. It's also important to note that social systems are a huge motivation behind exploration and puzzles as many NPCs give hints or advise the player to explore certain areas or find certain items hidden behind puzzles. Hidden information is a secondary system within social systems that adds a lot to the gameplay of TTYD. Interacting with the many characters in the game is almost essential to getting the full experience of what the game has to offer. Many characters and objects even offer information about the world and useful items and abilities that can be found or learned. A lot of the dialogue with in game characters creates a lot of charm and gives the feeling of a fleshed out world.
The last core system of the game is the combat system. This is the aspect of the game that gives a sense of master to the player. Strategy is vital in combat and the player must learn enemy weaknesses and strengths. Both social systems and movement actually play a significant role in the combat system. The more the player has utilized movement in exploration and finding hidden information within puzzles and in-game characters, their changes of success in combat go up. Strategy is a secondary system within the combat system. The player must learn that certain attacks give more damage or can stagger enemies but deplete energy very quickly. Enemies may have weaknesses to certain moves or abilities that will lower the difficulty of fights by quite a bit. A simple example would be to hit grounded enemies with the hammer to do good damage while enemies that fly can only be reached through jumping. The next secondary system is the use of power-ups. Some attacks power depend on how well the player performs in a short mini game before attacking. Other factors such as badges allow players to power up certain moves and abilities while outside of battle.