There were possibly going to be further sequels to Castlevania III: Dracula's Curse released on the NES, but due to the high sales and popularity of Konami's Teenage Mutant Ninja Turtles NES games, their development team was prioritized above all else leaving several other dev teams, including Castlevania's, to survive on the lesser profits their series generated.
During the boss fight on the third stage, you can see bursts of lightning in the clouds. If you throw a dagger at the Cyclops precisely at the point when a lightning bolt is about to strike down, its energy will collect into the dagger as it makes contact and thus electrocute the beast, taking off one quarter of its health.
Castlevania III's Japanese counterpart, Akumajou Densetsu, features a microprocessor chip in the cartridge, named the Konami VRC6. Aside from a few scrolling enhancements, the chip also adds three audio channels to the Famicom's existing five. These channels could provide more extensive and different kinds of sounds than the internal 2A03 chip. Since the international NES does not support the VRC6 expansion, a different chip was used, the Nintendo Memory Management Controller 5 (MMC5). This expansion, however, only adds two clone pulse wave channels rather than the VRC6's two different pulse waves and a sawtooth wave. As such, the NES version of Castlevania III does not feature the advanced sound that is in Akumajou Densetsu.
The international version of the game has many graphical differences when compared to the Japanese version. These include the cross in the intro losing its beams, the prologue clouds becoming static and the character name input screen's statues gaining robes.