so show me your mighty tricks how to move the city behind buildings (1st lvl)? 
you see how well they did this in ninja gaiden e.g.
This section would be the hardest to do from what I remember. We actually
did see how well they did that Ninja Gaiden bg effect in all the other PCE games that did it correctly. It only stands out in Ninja Gaiden because it's broken. Pretty much every stage in Lords of Thunder does it and uses the same method as Ninja Gaiden. Even if the buildings were to become more repeated (if on SCD instead of ACD), as long as it's smooth it'd blend in well with the rest of the screen looking so faithful overall. Higher skyscraper tiles on the screen could use different upper section art, building off of shared lower tiles. There are also small sections of the arcade lacking parallax (
like the far hallways of the final area) that the PCE port could add to with animated tiles.
Another
possible way, but not the one I'd go with would be to have the upper portion of the bg slide horizontally, with sprites overlapping. The non-boss sections of
are a good example of how much space sprites can cover a bg without noticeable flicker/breakup. I'd make the foreground garbage pile up higher in places and maybe add some other city distance art below the point where the taller buildings would scroll. Leading up to the buildings, the negative space between any new art could fade up into the buildings. The brown buildings could be shortened width-wise artistically if needed. Priority could be given for player sprites to flicker when characters jump at the top of the screen. Otherwise sprite bg sections would normally be above their heads. The invisible horizontal barrier where characters cannot walk above could be lowered a bit too. A variant of this method would require the HP bars, time, etc to either use strips of the bg/tiles or all sit above the play area. Again, I would do it with animated tiles (
if I was ever anything but an armchair programmer 
), but many PCE games pull this off surprisingly well.
or in the park with trees, toilets, lamps, fence etc.? and as i said the elevator wouldn't be possible as it is in the AC, SFC, MCD..
The park sections would be the easiest to do, with the upper portion of the screen sliding along with the odd sprite overlapping. The point where the arcade begins to scroll the top section could be moved up and the difference could be filled with more grass and tile tree bases. Reducing the frequency/space between stuff like trees could help reduce potential flicker. In the waterside night section, the distant bg doesn't need to scroll until above everyone's heads, so sprite overlapping wouldn't even be necessary below, but lowering the invisible play area ceiling could minimize potential flicker from characters jumping up in front of trees.
In the sunset section, the foreground giant lamps can be done with sprites, as it's been done in so many PCE games already (
they're nice and narrow until they're higher than anyone can jump). Aside from animated tiles for the fence, the whole bg below the horizon could be merged and the statue of liberty could move up a bit. The sky could then slide along with a sprite for the top of the statue of liberty. Again, even if the end result was
some parallax, but less overall than the Sega-CD version, people would still love it more for the actual Forgotten Worlds-quality graphics/art.
and btw. that elevator effect in riot zone looks awfull!! as are the other used tricks in riot zone to pretend parallax or such (e.g. rain).
You may not love the art style, but that effect worked perfectly. I never said that Riot Zone was a good example of parallax otherwise. The Final Fight elevator could be done mostly with animated tiles above the floor which could just push upward. The sections where the orange girders that are seen through the floor pushing upward could be depicted with animating tiles that push up with the rest of the floor. Even if the distant third bg that quickly disappears wasn't included, it wouldn't be a big deal. But it could be kept as it is in the Sega-CD version (merged with the girders) by filling in the gaps of the floor and back railing with natural looking art and having the the railing downward push upward. Alternately, the floor could stay the same art-wise, with the holes color cycling in-between girders until the distant third bg art is passed, while the girder behind the floor holes sections could still animate or just remain orange. If this port was pixel for pixel like Forgotten Worlds, the sides would be cropped, so the taller side rails wouldn't need to be worried about.
A SCD version wouldn't be
exactly the same, just as Forgotten Worlds wasn't (parallax aside). But like FW once you reach a certain level of faithfulness, people think they're looking at arcade-perfection anyway. It's easy now to compare different ports and arcade originals, but back in the day, people were blown away by much less than true 'arcade-perfectness'. A good example is the Sega-CD Final Fight which is heavily redrawn, but is considered by many to be perfect except for color. For a PCE Final Fight port, people would be pretty forgiving if a port of an often static bg arcade like Final Fight had
some sections become completely static if the rest of the game was sprinkled with various forms of parallax.
