The Sentinel is a computer game created by Geoff Crammond, published by Firebird in 1986 for the Commodore 64 and converted to BBC Micro (by Crammond himself), Amstrad CPC (with a cross-compiler written by Crammond), ZX Spectrum (by Mike Follin), Atari ST, Amiga (both by Steve Bak) and PC (by Mark Roll). It was released in the US as The Sentry. It was among the first games, if not the first, to do solid 3D on home computers. While it ran acceptably fast on 16-bit computers, it was notoriously slow on 8-bit machines such as the C64, where the next view took up to 3 seconds to be precomputed. Despite this, it was played by many fans for years, some of whom were able to modify their computers to enjoy it better (for example, by using a CMD SuperCPU in a standard 1-MHz 6502 Commodore 64 to achieve CPU clock speeds of 20 MHz).
The game itself can be best described as an "energy management" game. It has a first person point of view and features ten thousand levels. Its uniqueness caused it to be labelled "the first virtual reality game".
Graphically, the best version is the PC port, capable of VGA graphics and incremental lighting. This version, however, has very poor audio capabilities, since it only supports the PC speaker. The version with the best audio is the Amiga port, which features a soundtrack by David Whittaker.
A preview of a nonexistent sequel called Monolith appeared in 1995 in the Italian video game magazine The Games Machine as an April Fool's Day prank. In 1998, the real sequel called Sentinel Returns was released for PC and PlayStation; a freeware unofficial Sentinel clone called Sentry was also made available for PC the same year. In 2006, two more unofficial clones (Zenith and Sentinel) were released.
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:
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:
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 from 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.
It is clear that the memory limitations of the 8-bit microcomputers would preclude 10,000 landscapes being stored individually in the computer's memory. Instead, a procedural generation algorithm is used which generates each landscape from a small data packet, presumably the landscape's 4-digit number (since there are 10 unique digits and 10^4 = 10,000).