Some time ago, I drew a castle/monastery that I called Castle Dread. That map had a couple of access points to lower levels and here is one suggestion for what those might contain. I have a couple of thoughts about the rooms, but feel free to substitute as you wish.
First, the main burial chamber has been sealed off. The wall can be determined to be new with an easy perception/etc. check (adjust to your system). Inside you might have either an empty grave or a reanimated corpse of a noble.
Then there are a couple of secret doors. This part of the dungeon contains a few “preparation” rooms, a watch point overlooking the sea and a couple of holding cells. There is also an elevator leading down (I hope to draw the second level later).
Here is a black and white version, if you prefer that.
As usual, this map is free (see below), but if you want to tip me, check out my patreon page.
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