One of my arcade favourites is a game called Space Panic. Monsters is an exact reproduction of the original, right down to the sound effects.
You control a neatly drawn little man who runs around five floors linked by various configurations of ladders while being chased by monsters. As you might guess, the idea is to kill all the monsters as quickly as possible. Instead of a weapon, the little man is equipped with a spade with which he can dig and fill in holes. So he must dig a hole, wait until a monster falls into it, and then quickly fill the hole in before it can crawl back out. The unfortunate creature falls to the floor below and dies. The 'X' and 'Z' keys control horizontal movement, and '/' vertical, and 'D' and 'F' digging and filling.
In the first frame there are three red monsters, which look like huge tomatoes. If you kill all the monsters, a little tune is played and a new frame
appears. This time there are five red tomatoes. From the second frame onwards, any monster who manages to climb out of a hole mutates into a more devastating form. A red one turns green, and a green one turns white. The green and white ones are different, incidentally, and don't look at all like tomatoes.
Green monsters must fall through two vertically aligned holes, and whites through three. If any monster falls on top of another they are both destroyed. In addition to this, the green and white monsters are much smarter than the red ones and will follow you around relentlessly so that
it is difficult to find time to dig any holes.
Your oxygen level is shown by a red and yellow bar at the bottom of the screen; this gets progressively shorter throughout the frame. When it reaches red, the little man tires and eventually crashes to the floor and dies. This is not a real problem in the first few frames. Later on, when you may find yourself tackling several green and white monsters, it becomes vital.
As have said, Acornsoft's Monsters is high quality. The responses, graphics and sound effects are impeccable, making for one of the best games around.