The Sentinel

The Sentinel, released in the United States as The Sentry, is a puzzle video game created by Geoff Crammond, published by Firebird

The game can be best described as an "energy management" game. It has a first person point of view and features ten thousand playfields. Its uniqueness caused it to be labelled "the first virtual reality game".

The PC port supported VGA graphics, with an additional lighting effect: objects and terrain features become darker the farther away they are from the point of view. The Amiga port features a sampled soundtrack by David Whittaker.
In The Sentinel, the player takes the role of a Synthoid (called just "robot" in the US version), a telepathic robot who has to take control of a number of surreal, checkered landscapes of hills and valleys, by climbing from the lowest spot, where the hunt begins, to the highest platform, over which the Sentinel looms.

The Synthoid itself cannot move across the level; instead it can look around, accumulate energy by absorbing the objects that are scattered across the landscape, create stacks of boulders, generate inert Synthoid shells and transfer its consciousness from one of these clones to another.

List of executable actions:
Look around by moving the pointer on the screen (press S=Left, D=Right, L=Up, "Less-than sign"=Down)
Toggle cursor on|off. You turn faster with it off (press SPACE)
Absorb an object to gain its energy (point a square where an object is present and press A)
Create trees in empty squares (point the desired square and press T)
Create one or more boulders in empty squares (point the desired square and press B)
Absorb one or more boulders (point the boulder on the bottom of the stack and press A)
Create a new Synthoid shell in an empty square or on a boulder (point the desired place and press R)
Transfer consciousness to another Synthoid (point the Synthoid and press Q)
Hyperspace to a random part of the level at the expense of 3 units of energy (press H) (note that the player may not hyperspace to a higher square; only one of equal or lower height). Also used to hyperspace to the next level when the player has reached the Sentinel's height.
Rotate 180 degrees (press U)

Controlling Synthoids that are standing at a higher level is fundamental to the game, because only the objects which occupy a visible square can be interacted with (the player may absorb or create objects on a boulder if the sides can be seen). While doing so, the player must watch for the rotation of the Sentinel and be careful not to stand in an area which the Sentinel can see, or else it will start absorbing energy from the Synthoid, and when the energy is gone, the game is over.

Height is gained by placing a boulder on any visible square, and putting a Synthoid on the boulder. The player may then transfer consciousness to the new Synthoid, and absorb the old one. Stacks of boulders of any height may be created, if the player has enough energy. In order to absorb the Sentinel, the player must create a stack of boulders of sufficient height that the Synthoid on top can look down on the Sentinel's platform. When the Sentinel has been absorbed, the player may no longer absorb any energy from the landscape, although objects may be created as normal.

In later levels, the Sentinel is assisted by a number of Sentries. They behave exactly like the Sentinel, but absorbing them is not necessary to complete the level. Unlike the Sentinel, the Sentries do not stand on a platform but on ordinary squares. Attention must also be paid to nearby trees: if the Sentinel or Sentry cannot see the square the Synthoid is standing on, but its head is visible and there are trees in the vicinity, it may transform one of them into a Meanie, which will force the Synthoid to hyperspace and lose 3 units of energy. If the Meanie itself cannot see the player's square after a full rotation, it will turn back into a tree and the Sentinel or Sentry will resume rotation.

The rotation of the Sentinel and the Sentries is slow and predictable. However, if there are many Sentries, there will be few safe locations anywhere on the landscape. If either the Sentinel or the Sentries come across a source of energy (boulders or a synthoid), their rotation stops while they absorb the energy, one unit at a time. Meanwhile, to keep the total energy of the landscape constant, a tree is created randomly on the landscape for each absorbed unit of energy.

List of objects that can exist in the Sentinel world:
Tree (1 unit of energy)
Meanie (1 unit of energy)
Boulder (2 units of energy)
Synthoid (3 units of energy)
Sentry (3 units of energy)
Sentinel (4 units of energy)

A level is won by absorbing the Sentinel off its platform, creating a new Synthoid in the place of the Sentinel, transferring the consciousness to it and hyperspacing to a new level. When entering a new level, its number and an 8-digit code are displayed on the screen: these should be noted by the player, as entering them correctly is the only way to access that level again.

The number of levels that are skipped between two that are played depends on the amount of energy the player has accumulated when he jumps into hyperspace: the more energy he has, the more levels he will skip. Sometimes (depending on the individual skill of the player) it is necessary to replay a level in order to win it with less energy than the last time, because the difficulty of the levels is not incremental and one of them may be just too hard to complete