In this version of Tempest for the BBC, you have charge of a sentry ship on the borders of a one-dimensional universe (actually two) and are required to prevent the incursion of anti-matter aliens trying to make the transition from their galaxy to yours.
The PSS version of the game is a lot simpler graphically than the dedicated arcade game. Only one shape is used, a square set off-centre within a larger square. A number of lines connect pints between the two squares and two types of alien move out along these trying to reach the perimeter. The first type look like flashing arrows and are there only to provide points when you hit them. The second type are more sinister, and look like squashed Vs. When these reach the edge of the figure, they move round trying to settle on the same sector as your ship. If they succeed you lose one of four lives, but you do have access to two Star-Smashers at each level, and these will obliterate all aliens on display.
As you progress up the levels you must contend with intermediate sections of asteroid attack. This involves moving your ship around the perimeter avoiding fast-moving asteroids which again radiate from the centre of the figure. This is the hardest part.
The author of Sentinel has tried, where possible, to overcome the limitations of the BBC Micro (mainly speed) by sensible simplifications of the arcade game. There are a couple of minor grumbles, such as that you must complete a section to gain a score for it - losing a life sends you back to the start. The first section is also redundant since two bursts of the Star-Smasher will destroy everything.
Overall however, Sentinel is very enjoyable, addictive and a change from run-of-the-mill Space Invaders.