Excerpt from "Traveling the Beautiful Borgo Pass"
Whether you're a weary traveler looking for a quiet spot to enjoy some nature, or a demon out on the town, enjoying a bit of fun, it's hard to deny the allure of the river running through Dracula's lands. This gorgeous strip of water runs through the lengths to domain, cutting a blue swath across the low country. Populated by scenic bends, faster rapids, and a glorious waterfall (or five), this river is majestic as the creatures living in it are dangerous.
But then a little danger only adds to the fun of a trip. Is it really a vacation if you haven't run screaming into the night at least once. It makes for memorable stories to tell your grandkids (if you survive).
About the River:
Before we can discuss the River we have to define what it is. Within the scope of Dracula's lands the river is an external body of water that hasn't been built over with stone (which would make it an aqueduct) and exists above ground (so as not to be an underground waterway or a water feature within caverns). Within that context (as there are other stages with water features that will be covered elsewhere) we have a few stags that do meet those qualifications.
The first actually has to be discuss with an asterisk as the Dead River (and, by extension, the Yuba Lake) are bodies of water that technically meet our definition. They are key locations that further hero Simon Belmont on his adventure to collect the body parts of Dracula and break the curse hanging over the hero. These areas (as with the rest of Castlevania II: Simon's Quest) reside outside of Dracula's lands, though (at least by the strictest interpretation of how far his lands extend) and thus aren't really part of Castle Dracula or its domains.
Within the bounds of Dracula's true lands we have to move forward to Castlevania Dracula X: Rondo of Blood for the alternate exit to Stage 4', Fortress of the Water Demon. Here our heroes have to drop into a specific hole in the hills of the stage, dropping into a fast flowing river that will take them down to a lower , water-filled area. This will then lead them into boss chamber where the heroes will have to take on the vampiress Carmilla and her vassal Laura. it is worth noting that this section evokes a similar flow and design to the Aqueduct in Super Castlevania IV, although that doesn't technically count as a proper river.
From there we don't actually get another aboveground river in any of the 2D entries in the series. We do get the Foggy Lake in Castlevania 64, although that section is really built around the Ghost Ship that acts as the central feature of the stage. Then, much later, there was Oblivious Lake in Castlevania: Lords of Shadow), although that water feature was frozen over and really only acts as a set-piece for the battle against the Ice Titan.
Fortress of the Water Demon, river section, in Castlevania Dracula X: Rondo of Blood