Viewpoint

View Points are any structure high enough for it to be possible to observe the surrounding terrain. They range from church towers and mansions' roof beams in the cities, to watchtowers and guardposts on the roads. They help reveal the map to the protagonist.

Assassin's Creed
In the first game, view points were found in all the cities (Damascus, Jerusalem and Acre) and in the Kingdom, excluding Masyaf. Each map contained 10 to 12 view points located on the highest buildings; such as a cross on top of a Catholic Cathedral, or the top of a lighthouse in the port-city of Acre, or a palace guard tower in the city of Damascus. Aside from by the height of those constructions, they could be located by watching the birds that nest there. Climbing atop one of the structures provided the means to activate (or Synchronize) the View Point, using the Head Button.

Those view points allowed Altaïr to observe the layout of the terrain around him and watch the movement of its people. After the first view point in a map was "activated," it revealed the location of the remaining points and missions within a certain radius.

Some viewpoints had archers defending the position, or guards set around and below it. They could either fire arrows or throw rocks to prevent a climber from reaching the top. When the player ascends to the synchronization point of the building, often an eagle will be seen flying around the viewpoint, occasionally stopping to perch itself on the viewpoint. All the view points have carts or piles of hay on the ground at the very bottom, providing perfect and spectacular locations for the performance of an incredible-but-potentially-fatal Leap of Faith.

Achievement
On the Xbox 360 version you can get an achievement by synchronizing with all View Points.

Assassin's Creed II
Assassin's Creed II's view points function much the same and some are located on famous land marks of Italy. Ezio will not only be able to dive into carts and stacks of hay such as in the first game, but also into carts of flower petals, leaves and bodies of water. There are seventy-three View Points in the game (66 not including DLC's) and you get an achievement/trophy ("I like the view") by synchronizing with ten View Points. You can also get the achievement "High Dive" where you perform a leap of faith on top of Giotto's Campanile in Florence.

Note that one's map will remain devoid of structures until the player has ascended to the View Point in the area and synchronized with it.

Trivia

 * In the achievements for Assassin's Creed, the "Fearless" achievement refers to View Points as "Reach High Points".
 * The highest point that you can reach in Assassin's Creed is the Cathedral of the Holy Cross in Acre.