Hey, I should've explained better in the game, my apologies. If you speak to the NPC in the dungeon, they'll tell you you have to press 3 buttons hidden around the mansion to open the door to the Count's chambers. The bullets are an optional collectible; there are 3 in the first area and 3 in the second.
Funny you mention that, I actually made the levels in TrenchBroom and imported them with Qodot! So the Quake comparison is pretty accurate haha. I hope you enjoyed!