Assassin's Creed is a third-person stealth game in which the player assumes the role of Altaïr ibn La-Ahad (الطائر ابن لا أحد , Arabic, "The Flying One, Son of None"), a member of the Hashshashin (The Assassin Brotherhood) which is not allied to either the Templars or Saracens during the third crusade. Altaïr's objective in the game is to assassinate nine historical figures propagating the Crusades in the year 1191. (According to Ubisoft developers, all of the main character's targets are historical figures who died or disappeared in 1191, although not necessarily by assassination.)

The player must climb tall towers in order to map out the city and discover the location of investigation targets.
The primary goal of the game is to carry out the assassinations ordered by the head of the Brotherhood, Al Mualim. To achieve this goal, the player must use stealth and a variety of intelligence gathering tactics to collect information on their target.
These tactics include eavesdropping, forceful interrogation, pickpocketing, and completing tasks for informers, fellow assassins who will give you information in exchange for assassinating targets or collecting flags. Additionally, the player may take part in any number of side objectives, including climbing tall towers to map out the city and saving citizens that are being threatened or harassed by the city guards. There are also various side quests that do not advance the plot such as hunting down and killing Templars and flag collecting.
The game takes place primarily in the Holy Land. It consists of four main cities: Jerusalem, Acre, Damascus and Masyaf, the assassins' stronghold; all but Masyaf contain three sections that are unlocked over the course of the game. An approximation of the land lying between these areas is present in the game as "The Kingdom." The game's developers claim the environments are all historically accurate. Each city features a highly detailed environment populated by many people of different types, and as the player interacts with his/her environment, the people around Altaïr respond in logical and predictable ways. For example, when Altaïr climbs buildings without the use of a ladder, the local citizens pause, gather round the building he is climbing and comment on his unusual behavior, or how they won't help him if he hurts himself. Certain actions, such as harming an innocent can attract the unwanted attention of nearby guards, who may then choose to attack him. Neither Richard the Lionheart's forces, nor Saladin's forces are particularly friendly towards Altaïr - If he comes into contact with any guards outside the main cities, he will be chased off, no matter which side.

Sitting on a bench will allow the player to blend in and avoid being spotted by guards.
The player is made aware of how noticeable Altaïr is to enemy guards as well as the current state of alert in the local area. To perform many of the assassinations and other tasks, the player must consider the use of both "low profile" and "high profile" commands. "Low profile" commands allow Altaïr to blend into nearby crowds, gently pass by other citizens, or other non-threatening tasks that can be used to hide and reduce the alertness level. "High profile" commands are more noticeable, and include running, scaling the sides of buildings to climb to higher vantage points, and attacking foes; performing these actions at certain times may raise the local area's awareness level. Once the area is at high alert, the player must use both low profile commands as well as aiming to climb away from ground level or using hiding spaces, such as benches, rooftop gardens, haystacks or by disgusing himself amongst scholars, to reduce the alertness level. The player, as they complete the assigned assassinations, will gain either new weapons or new combat skills including counter-attacks and attack dodges. Besides his fists, Altaïr's weapons include an assassin's sword for standard combat, a short blade, and throwing knives which are used in conjunction with the short blade.
Altaïr also starts the main storyline with the use of a hidden retractable blade on his left arm that can be used for killing targets at very close quarters without creating any immediate alerts (although, if others come across the body, the alertness level may be raised). This is the most noticeable difference between low and high profile. A low profile assassination can be performed within as little as a few feet of other guards or soldiers without attracting attention, whereas a high profile assassination attack will be noticed by everyone within eyesight. This becomes more difficult as the game progresses, as guards become more attentive for Altaïr's presence due to his repeated killings.

During gameplay, intentional glitches that include organic compounds and computer messages will flash on the screen.
The player's health is described as the level of synchronization between Desmond and Altaïr's memories; should Altaïr take damage, some amount of synchronization is lost, and if all synchronization is lost, the current memory that Desmond is experiencing will be restarted at the last checkpoint. When the synchronization bar is full, the player has the additional option to use "eagle vision" which allows the computer-rendered memory to highlight all visible characters in colors corresponding to whether they are friend or foe or even the target of their assassination.
Due to Altaïr's memories being rendered by the computer of the Animus project, the player may experience "glitches" in the rendering of the historical world, which may help the player to identify targets, or can be used to alter the viewpoint during in-game scripted scenes should the player react fast enough when they appear.