Akiropito, who did music, writing, casting, and sound driver programming for this game, has been kind enough to post a lot of interesting information about its development on Twitter. You can read it all here:
twitter.com/akiropito2nd
I hope everyone has been doing well after 32 years...
(13/13)

In Europe, it would have been imported through Telegames. In this catalog from December 2000, it's listed at £149.99! Way more expensive than any of the other games, which shows how high the demand must have been back then. Nowadays, some of those other games are worth several times that amount

It seems to have been a popular import title at the time of release given that the TurboGrafx CD was region free. On OneyPlays Dave mentions that Tom Fulp had it, and on Game Sack Joe mentions having ordered it from Diehard Game Club, an importer that had a few locations in the US

The game uses Roland Sound Space which simulates 3D audio with stereo. I was really caught by surprise at how good it sounds when I first heard it coming from speakers plugged into the Duo's 3.5mm jack, listening to the music on YouTube just doesn't do it justice

Several spots that are usually impossible to reach still have proper collision, like this infamous ledge in stage 1 for example. Sadly, there's no collision at the bottom of the ship in stage 5, even though it can be reached normally with a well-timed double jump

Everyone already knows this, but the key is the most powerful weapon in the game, it deals the most damage and doesn't use any hearts. I wonder if a key-only run is possible, probably not without damage boosting because I think the range is too poor to hit some enemies

The font on the score pickups is slightly different for each player, because obviously the game would have been completely unplayable if this weren't the case

I like how the lightning effect in stage 6 actually takes into account the form of the pillars in the background. Also, I just noticed Maria's in-game sprite looks nothing like her design in the cutscenes and promotional art, it seems to be based on her old concept art shown in magazines.

For some reason, Richter's sprite changes to this cool lil' fella while the game is loading. The graphic is actually loaded at all times in extra space at the end of ADPCM RAM. Maria has more voice samples and thus no space to store graphics there, so her equivalent sprite isn't as interesting

To reduce CPU usage new particles don't spawn if there are already too many on screen, as opposed to the approach in Super Mario Bros where old particles just despawn. Similarly, you can't item crash while a subweapon is active because the item crash graphics use the same VRAM space as the subweapon

Rather, this game just does an outstanding job at hiding the limitations of the PC Engine's weak graphics hardware and 8-bit CPU. Despite only a single background layer, there are so many parallax scrolling effects such as in this scene where all the trees in the foreground are drawn with sprites

Being released on a console whose library primarily consists of simple arcade and adventure games, it's no surprise that many people who've played this game don't know or even care what a PC Engine is. Is "Circus Lido" too silly to appeal to fans of a franchise as serious and mature as Castlevania?