The back of The Sentinel box says:
Beyond your wildest dreams, in a world where the only force is pure energy, stands the Sentinel. Battle against him through 10,000 lands, in the most original, compelling and addictive computer game ever devised. Firebird issue the challenge, and the Sentinel awaits.
You can see the entire instruction booklet by visiting the Firebird Downloads section.
The Sentinel was initially released for the C64, BBC Model B, Amstrad CPC and ZX Spectrum in 1986. The 16-bit versions for ST, Amiga and PC followed in 1987 and 1988.
The cassette versions were released in the standard crystal cases, complete with a small fold out colour instruction booklet that described the rules of the game.
The box artwork was by David J. Rowe and was one of the most memorable paintings ever done for a TelecomSoft game.
The ST & Amiga conversions were included as part of a more general 16-bit advert for Firebird, also featuring Black Lamp, Return to Genesis and Pandora as well as The Sentinel. The IBM PC version was never advertised.