The game begins with the protagonists Rikimaru and Ayame training to reach an honourable rank. In the training missions the performance of the character determines their rank; ranging from Thug, Novice, Ninja, Master Ninja to Grand Master. Lord Gohda himself gives his feedback on Rikimaru's performance while his daughter Princess Kiku speaks to Ayame instead. The playable characters Rikimaru and Ayame are only selectable as singular players and their storylines vary considerably, yet they experience the same levels and missions. Rikimaru, the oldest of the two, is the primary character in the game. Armed with a single ninjato, he is stronger than Ayame but relatively slower. Ayame carries a pair of tantō (although she does carry dual wakizashi in the second game) and is faster and has more combos than Rikimaru, but is the weaker of the two.
The game features 10 levels which are introduced in increasing difficulty: Punish the Evil Merchant, Deliver the Secret Message, Rescue the Captive Ninja, Cross the Checkpoint, Execute the Corrupt Minister, Infiltrate the Manji Cult, Destroy the Foreign Pirate, Cure the Princess, Reclaim the Castle and Free the Princess. All of the levels take place at night to compensate for the technical limitations of the PlayStation; the game's high rate of redraw is reduced by setting events at night and reducing the distance the player can see. Both characters are armed with a grappling hook which allows them to zip up to the tops of buildings and move freely across rooftops, which gave the game a sense of verticality that was rare among most action/adventure games of the time. Many bosses react differently to the two ninja based on their genders, highlighting the social attitudes at the time.
All missions, including the initial training mission, can be replayed for an unlimited number of times, but Mission 2 and onwards cannot be accessed without the previous chapter being successfully completed. Three different layouts are used to distribute the enemies, obstacles and items spread on the levels' maps. On playing the training level, only several throwable deadly objects and a grappling hook are automatically selected as interactive objects. The weapon selection section contains throwing knives (kunai)/shuriken (depending on the game version), smoke bombs, caltrops, poison rice cakes, coloured rice, healing potions, grenades and mines. Advanced items on completing a mission with the rank Grand Master are known as: Super Shuriken/Kunai, Lightfoot Scroll, Fire Eater Scroll, Protections Amulet, Sleeping Gas, Ninja Armour, Shadow Decoy, Resurrection Leaf, Chameleon Spell, Dog Bone and Decoy Whistle. These are optional to use on missions but are limited to a number of objects overall. The Grappling Hook is permanently locked into the inventory and does not consume item space.
On failure, the character dies (sometimes being ridiculed by a boss) and the text "Mission Failed" appears on the screen in front of the grave of the respected character. All items that he/she acquires or takes with him/her are permanently lost until found in future missions, therefore the player should note a risk in selecting a large number of objects to use for the mission. If the character succeeds in their mission they will be scored and ranked while the player's time is recorded into the game's system. This score is compared to previous attempts and the scores of the partner to the current character if they have been played before. A cinematic scene happens at the end of each level revealing some of the character's personality to the player.
Mission 10 has a longer cutscene which leads to the ending of the game and credits. The player still has an option to complete the game as the alternative character as the attempts in playing each level are limitless. Some aspects can be unlocked by completing sections of the game, which can include alternative costumes for Rikimaru and Ayame. Cheats can also trigger unlockables.
The game takes place in feudal Japan, introducing a pair of ninja: Rikimaru (力丸) and Ayame (彩女), members of the Azuma ninja clan since childhood. The two ninja serve the heroic Lord Gohda, and work for him as his secret spies to root out corruption and gather intelligence in his province. However, the evil demonic sorcerer Lord Mei-Oh seeks to destroy Lord Gohda, and using his demon warrior Onikage who wreaks havoc throughout Lord Gohda's province.
A video of an early prototype build of the game was featured in Tenchu: Shinobi Hyakusen, showing a science-fiction type ninja game whose gameplay style was also very different from the one in the final version. Famous ninja-roles actor/martial artist Sho Kosugi and his son Kane were hired as motion capture actors for the game's combat moves.
The Japanese version of the game is somewhat different from the Western version and the remake Shinobi Gaisen. The game came with 8 levels (missing Cross the Checkpoint and Punish the Corrupt Minister) and each level only has one layout. The option screen, character select and level select all come with different graphics. The boss characters have different weapons and before each level there is a small amount of Japanese dialogue that gives more detail into the game. The fighting moves are also different, whereas on the later version, mashing the button for attacks is all well and good, but in this version each strike must be timed to get the full flurry of moves. The 180 degree reverse roll is also absent.
Tenchu was re-released in Japan on February 24, 1999 with many updates, including different mission layouts, all the stages seen in the American and European versions (levels 4 and 5 were missing in the original) and four selectable languages: Japanese, English, Italian and French.
The main feature of this version was its mission editor. The possibilities in creating missions were endless and it generated a special edition disc with the 100 best missions created by Japanese players called Tenchu: Shinobi Hyakusen.
Tenchu: Shinobi Hyakusen is an expansion pack for Tenchu. It was released on November 11, 1999 for the PlayStation. It is a standalone expansion, meaning it does not require the disc for the main game.
The game is based on the levels created by the users of Shinobi Gaisen. Acquire held a competition for these levels, and the best one hundred missions were collected, and ultimately formed Tenchu: Shinobi Hyakusen. There are also secret missions to unlock. The whole game consists of 122 missions. The theme of each mission focuses on a chosen objective. Some missions emphasise elimination, while others focus on assassination, finding a peculiar flower or protecting Princess Kiku along the way.
The game fundamentals and controls remained unchanged. However, missions in Shinobi Hyakusen are not bound together by any storyline. Playing through the missions earns the player FMV and other promotional videos, beta test captures, artwork and other behind-the-scenes material otherwise not accessible.
Tenchu: Stealth Assassins received "favorable" reviews according to video game review aggregator Metacritic. It was ranked as the 54th top game of all time by the staff of Game Informer in 2001: "Tenchu: Stealth Assassins showed the gaming world that it takes more than just dark clothes and pointy throwing objects to make it as a ninja. Forcing players to learn and utilize stealth techniques to not only excel, but merely survive, Tenchu is a challenging, nerve-wracking game that leaves you screaming in frustration, then crawling back for more."