The series was intended to continue with Sword of the Samurai and three more games, but due to low sales the series never made it beying Temple of Terror.
The game engine was licensed from Roger Taylor's Ashminster Computing.
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Both the C64 and ZX Spectrum version of this game is fatally bugged and cannot be completed. (At least I haven't seen a good copy.) I'm working on a C= Plus/4 conversion and had to debug the game because of this. As the solution listed here mentions, inserting any of the final 4 items into the locked door results in a weird message (about the pirates falling into the river).
The game internally uses variables, and variable $C is used for both flagging the pirates falling from the bridge, and counting the number of items inserted into the door. The code does check if the variable $C reaches 4 (all items inserted), and then it would open the door, but this can never happen, because the earlier code that deals with the pirates always clears the variable (sets it back to zero).
All of this is pretty much "set in stone", so it's not like it's a small typo or anything - I'd say the game was not tested before release, otherwise they would have noticed this glaring error.
Buggy and dull game - I'm fairly sure I made it to the end, but it is without a doubt the worst title in the series.