The moral of the game is that you're stronger as part of a team than as individuals. You would assume that would mean that you use one of the dual attacks in the final battle, but you'll quickly get wiped out if you do this, because Dracula has learned the moral of the game as well. So if unity is strength, how do you defeat an enemy? By dividing and conquering; since Death can fly and Dracula cannot, all you need to do is run up to one of the higher platforms in the room, Death will get there much faster than Dracula, allowing you to take him out with the Cross sub-weapon. Dracula is then a piece of cake, since they were far stronger together than apart.
The entire plot of Portait of Ruin revolves around a powerful vampire, Brauner, usurping Dracula's power by sealing Dracula away from his castle. We also know that sealing Dracula away from his castle before killing him is what ultimately killed him for good about 50 years later. Thus the brilliance comes in — it was probably the events of Portrait of Ruin and Brauner's seal that gave the Belmonts both the idea and the means (though not exactly the same method) to seal Dracula's castle away from Dracula and thus finish him once and for all, meaning Brauner's plot unintentionally ended up saving the world in the end.
One interpretation of the Nation of Fools portrait is that the ruined carnival featured is where Brauner lost his daughters in World War I. World events, however, paint another possibility. Up until 1944 — the year Portrait of Ruin takes place in — Romania had been under a fascist government that embraced Nazism between World Wars, executed up to 380,000 Jewish Romanians and 11,000 Roma (gypsies), and sent another 1.2 million Romanians to their deaths in Nazi Germany's doomed campaign against Soviet Russia. The vampire Brauner is a fictionalization of a real-life Romanian-Jewish surrealist that lived during that time. The portrait world is a gypsy carnival being destroyed from within. Its boss is Legion, a monster encased in a shell of corpses at the heart of the carnival's destruction. It's a surrealist representation of what Castlevania!Brauner's seen his home country become: a Nation of Fools.
Also within Portrait of Ruin, we have how you first meet the Sisters and Brauner. You first meet Loretta in the City of Haze after destroying Dullahan, and you meet Brauner and the Sisters in the Sandy Grave Portrait. And yet both times, they seem to check up on the state of things or seem outright surprised to see Johnathan and Charlotte there. Why is this? In Sisters Mode, which serves as a prologue to Jonathan and Charlotte's story, you were required to go through the paintings and destroy the same bosses that Johnathan and Charlotte fight. Each boss destroyed weakens Brauner's hold on the castle. Sadly, when the Sisters get to Brauner, it's here that they get turned into vampires and brainwashed. The reason they were probably checking up/surprised to see Johnathan and Charlotte within the portraits is because Brauner is fixing the damage the Sisters did and making sure they uphold, re-establishing his hold on the castle by bringing the previously destroyed bosses back.
One of the stronger weapons in this game is a whip made of a rose stem. This is not just the idea of thorns being painful. Roses were traditionally believed to have the power to ward off evil. In fact, in the original Dracula novel, while it doesn't come to pass, Van Helsing does comment that he wanted to try using rose thorns on Dracula to see if it would work.
Similarly, the knife subweapon probably originates from the knife used to kill Dracula in the original book.
This game brings up the reason why the Vampire Killer is not usually seen in the hands of non-Belmonts: using the whip too much can and will outright kill the non-Belmont user. And yet why does it have this quality? Well, taking a peek back at Lament of Innocence reveals that it was originally a Blood Pact between Sara and Leon, where in Rinaldo's words "[Leon's] blood accepts your hatred to slay your kind". Leon's blood which would be the Belmont Bloodline. Anyone who isn't of the Belmont Blood has not accepted Sara's hatred, and thus is forced to bear the brunt of it as diminishing their very lifeforce in exchange for the awesome power Vampire Killer has to give.
The Nation of Fools. It's mentioned that Brauner lost his daughters because of the first World War. The painting (and by extension, the stage) shows the aftermath of a circus and town torn apart by war. Put two and two together. Worse, as you go further into the level, the stage gets turned on its side, and then upside down, representing how Brauner's life was literally turned upside down. And then there's the boss...
