Triffix‘s Castelian is a conversion of the classic Nebulus by John M. Phillips. And it is a top little conversion!
The monochrome graphics work well, reproducing the iconic ‘revolving cylinder’ effect on the towers, which you must climb to ‘beat’, in gameplay terms.
The original, highly devious gameplay has also been translated well, although there are a couple of differences to the original. The submarine sections have been replaced by a side-scrolling platform ‘bonus’ section, and the main character is no longer called “Pogo” but “Julius”. Harumph!
In spite of that Castelian is a classic game that has withstood the conversion process well and is one of the better games on the Nintendo Game Boy.
More: Nebulus on Wikipedia