Yeah, it's probably combining the R3 trick with two sets of tiles (displaced by one pixel). It doesn't seem to have a big tile variety at all. In my case I just can't afford that technique, I can barely fit in memory the 256 different tiles per level in my game (I already use 3 screen buffers, tilemap, sprites, soundtrack...) . It would require to double the tile memory, which is out of consideration.
A further consideration with using preshifted tiles like that is that the leading edge column of pixels of a tile needs to be identical to the trailing edge column of pixels of the preceding tile, so that needs to be dealt with in some way, and is likely to either cost cpu or limit tile arrangement or design.
could a TATE mode work well on the CPC? You could then leverage off the horizontal hardware scroll, potentially full screen
I think there are a couple of other things working against a TATE mode as well.
Primarily it's that with a standard orientation vertical scrolling screen sized to 32 characters wide, you have pretty much the ideal for fast screen addressing, with each line being an even 64 bytes long with no concerns about memory page changing in a pixel line, let alone screen address reset. With a horizontal scroll it's the opposite, not only do you need to worry about a line of pixels having page changes, but also dealing with the position where the screen address resets and goes backwards by 2kb. So it would complicate and slow screen addressing.
A small secondary possible issue, depending on your point of view, is that a score line is relatively easy to maintain as a screen split in the standard orientation below the play area, where if you were using a horizontal scroll, your only easy option would be a 'vertical' score line along the left or right side of the play area, which I suspect would not really achieve the 'arcade feel' I imagine a TATE mode would be aiming for. There's options you could use for putting a score at the top or bottom of the screen, but the ones I can think of at the moment would use up more cpu time than a 'vertical column to the side' style score panel.
As abalore says, that doesnt mean it's impossible, but I think there is a lot more going for using a vertical scroll in standard orientation on CPC.
