Segues, no... but here's some area-based ideas.
But you definitely need dungeons. They can also give way to catacombs. You already mentioned a Clock Tower, which I totally support, and also suggest you put in a giant clock (possibly with a puzzle). A haunted/possessed/otherwise-corrupted chapel could also fit in just fine.
You might benefit from looking into some Ravenloft-style material.
You already mentioned Vampire (to keep with the Castlevania style), but alternatives... Rakshasa Sorcerer/Lich, Nightwalker, regular Lich, Angel of Decay (Libris Mortis), and they would all fit well with a Negative Energy Plane portal.
I suppose I should also ask these questions: What level do you expect the party to be for this game? And, are you going for more Horror (with a touch of action) or Action (with some creepy stuff thrown in)?