diff --git a/openslide/__init__.py b/openslide/__init__.py
index 8994bcdf..60a32b41 100644
--- a/openslide/__init__.py
+++ b/openslide/__init__.py
@@ -25,6 +25,7 @@
 
 from __future__ import annotations
 
+from abc import ABCMeta, abstractmethod
 from io import BytesIO
 from types import TracebackType
 from typing import Iterator, Literal, Mapping, TypeVar
@@ -62,7 +63,7 @@
 _T = TypeVar('_T')
 
 
-class AbstractSlide:
+class AbstractSlide(metaclass=ABCMeta):
     """The base class of a slide object."""
 
     def __init__(self) -> None:
@@ -81,22 +82,26 @@ def __exit__(
         return False
 
     @classmethod
+    @abstractmethod
     def detect_format(cls, filename: lowlevel.Filename) -> str | None:
         """Return a string describing the format of the specified file.
 
         If the file format is not recognized, return None."""
         raise NotImplementedError
 
+    @abstractmethod
     def close(self) -> None:
         """Close the slide."""
         raise NotImplementedError
 
     @property
+    @abstractmethod
     def level_count(self) -> int:
         """The number of levels in the image."""
         raise NotImplementedError
 
     @property
+    @abstractmethod
     def level_dimensions(self) -> tuple[tuple[int, int], ...]:
         """A tuple of (width, height) tuples, one for each level of the image.
 
@@ -109,6 +114,7 @@ def dimensions(self) -> tuple[int, int]:
         return self.level_dimensions[0]
 
     @property
+    @abstractmethod
     def level_downsamples(self) -> tuple[float, ...]:
         """A tuple of downsampling factors for each level of the image.
 
@@ -116,6 +122,7 @@ def level_downsamples(self) -> tuple[float, ...]:
         raise NotImplementedError
 
     @property
+    @abstractmethod
     def properties(self) -> Mapping[str, str]:
         """Metadata about the image.
 
@@ -123,6 +130,7 @@ def properties(self) -> Mapping[str, str]:
         raise NotImplementedError
 
     @property
+    @abstractmethod
     def associated_images(self) -> Mapping[str, Image.Image]:
         """Images associated with this whole-slide image.
 
@@ -136,10 +144,12 @@ def color_profile(self) -> ImageCms.ImageCmsProfile | None:
             return None
         return ImageCms.getOpenProfile(BytesIO(self._profile))
 
+    @abstractmethod
     def get_best_level_for_downsample(self, downsample: float) -> int:
         """Return the best level for displaying the given downsample."""
         raise NotImplementedError
 
+    @abstractmethod
     def read_region(
         self, location: tuple[int, int], level: int, size: tuple[int, int]
     ) -> Image.Image:
@@ -151,11 +161,13 @@ def read_region(
         size:     (width, height) tuple giving the region size."""
         raise NotImplementedError
 
-    def set_cache(self, cache: OpenSlideCache) -> None:
+    def set_cache(self, cache: OpenSlideCache) -> None:  # noqa: B027
         """Use the specified cache to store recently decoded slide tiles.
 
+        This class does not support caching, so this method does nothing.
+
         cache: an OpenSlideCache object."""
-        raise NotImplementedError
+        pass
 
     def get_thumbnail(self, size: tuple[int, int]) -> Image.Image:
         """Return a PIL.Image containing an RGB thumbnail of the image.
@@ -299,6 +311,7 @@ def __len__(self) -> int:
     def __iter__(self) -> Iterator[str]:
         return iter(self._keys())
 
+    @abstractmethod
     def _keys(self) -> list[str]:
         # Private method; always returns list.
         raise NotImplementedError()
@@ -406,7 +419,7 @@ def level_dimensions(self) -> tuple[tuple[int, int]]:
 
         level_dimensions[n] contains the dimensions of level n."""
         if self._image is None:
-            raise ValueError('Passing closed slide object')
+            raise ValueError('Cannot read from a closed slide')
         return (self._image.size,)
 
     @property
@@ -444,7 +457,7 @@ def read_region(
         level:    the level number.
         size:     (width, height) tuple giving the region size."""
         if self._image is None:
-            raise ValueError('Passing closed slide object')
+            raise ValueError('Cannot read from a closed slide')
         if level != 0:
             raise OpenSlideError("Invalid level")
         if ['fail' for s in size if s < 0]:
@@ -474,14 +487,6 @@ def read_region(
             tile.info['icc_profile'] = self._profile
         return tile
 
-    def set_cache(self, cache: OpenSlideCache) -> None:
-        """Use the specified cache to store recently decoded slide tiles.
-
-        ImageSlide does not support caching, so this method does nothing.
-
-        cache: an OpenSlideCache object."""
-        pass
-
 
 def open_slide(filename: lowlevel.Filename) -> OpenSlide | ImageSlide:
     """Open a whole-slide or regular image.
diff --git a/openslide/lowlevel.py b/openslide/lowlevel.py
index 48963057..c24ca7c9 100644
--- a/openslide/lowlevel.py
+++ b/openslide/lowlevel.py
@@ -568,7 +568,6 @@ def read_associated_image_icc_profile(
     'openslide_set_cache',
     None,
     [_OpenSlide, _OpenSlideCache],
-    None,
     minimum_version='4.0.0',
 )