Must say I'm confused by Baf's review. I played this (on Spectrum 48k) when I was young, and though it had some difficulty, it is all logical, and I was happy as Larry when I completed this - my first ever solved adventure game - in an age when walkthroughs and solutions were almost non-existent. I would say it is of average or less difficulty level. Of course it is constrained by the inputs of the time, which were a little less forgiving in the 1980s, and it is true to say that it is not as complex as other of Scott's efforts.
It really draws you into the spy world though, and you become eager to get through all doors possible. Make sure if playing Spectrum version that you hit ENTER, as some old Brian Howarth / Scott Adams games required you to remove the graphics to get all the info. Parser is ok, and accepts commands such as GET ALL (as I recall).
If only more games allowed use of the FRISK command...