diff --git a/overviewer.py b/overviewer.py
index be41d5a..a5a8498 100755
--- a/overviewer.py
+++ b/overviewer.py
@@ -277,16 +277,22 @@ dir but you forgot to put quotes around the directory, since it contains spaces.
     if not os.path.exists(destdir):
         os.mkdir(destdir)
 
+    # saves us from creating the same World object over and over again
+    worldcache = {}
+
     for render_name in render_things:
         render = render_things[render_name]
         logging.debug("Found the following render thing: %r", render)
 
-        # XXX we now construct the regionset directly
-        #w = world.World(render['worldpath'])
+        if (render['worldname'] not in worldcache):
+            w = world.World(render['worldname'])
+            worldcache[render['worldname']] = w
+        else:
+            w = worldcache[render['worldname']]
 
-        # if no dimension has been specified, just use the first one
-        # TODO support the case where a different dimension is specified
-        rset = world.RegionSet(render['worldname'])
+
+        # TODO get the correct regionset based on render['dimension']
+        rset = w.get_regionset(0)
         logging.debug("Using RegionSet %r", rset) 
 
         # create our TileSet from this RegionSet
diff --git a/overviewer_core/configParser.py b/overviewer_core/configParser.py
index 7fbef0b..a2f6601 100644
--- a/overviewer_core/configParser.py
+++ b/overviewer_core/configParser.py
@@ -263,10 +263,6 @@ class MultiWorldParser(object):
 
     def validate(self):
 
-        # first validate the world dict
-        for worldname in self.world:
-            if not os.path.exists(self.world[worldname]):
-                raise Exception("%r does not exist for %s" % (self.world[worldname], worldname))
 
         origs = dict()
 
diff --git a/overviewer_core/settingsDefinition.py b/overviewer_core/settingsDefinition.py
index 7864f36..7d74a37 100644
--- a/overviewer_core/settingsDefinition.py
+++ b/overviewer_core/settingsDefinition.py
@@ -18,6 +18,7 @@ from settingsValidators import *
 
 render = {
     "worldname": dict(required=True, validator=validateWorldPath, save_orig=True),
+    "dimension": dict(required=False, validator=validateDimension, default="default"),
     "title": dict(required=True, validator=validateStr),
     "rendermode": dict(required=False, validator=validateRenderMode),
     "northdirection": dict(required=False, validator=validateNorthDirection),
diff --git a/overviewer_core/settingsValidators.py b/overviewer_core/settingsValidators.py
index db10815..91e172b 100644
--- a/overviewer_core/settingsValidators.py
+++ b/overviewer_core/settingsValidators.py
@@ -9,7 +9,10 @@ def validateWorldPath(name, **kwargs):
     world = kwargs.get('world', dict())
     if name not in world.keys():
         raise ValidationException("bad world name")
-    return os.path.abspath(world[name])
+    abs_path = os.path.abspath(world[name])
+    if not os.path.exists(os.path.join(abs_path, "level.dat")):
+        raise ValidationException("No level.dat file in %r.  Check the path for world %r" % (abs_path, name))
+    return abs_path
 
 
 def validateRenderMode(mode, **kwargs):
@@ -94,3 +97,9 @@ def validateInt(i, **kwargs):
 
 def validateStr(s, **kwargs):
     return str(s)
+
+def validateDimension(d, **kwargs):
+    if d in ["nether", "overworld", "end", "default"]:
+        return d
+    raise ValidationException("%r is not a valid dimension" % d)
+
diff --git a/overviewer_core/world.py b/overviewer_core/world.py
index 82c3a69..8ff3e68 100644
--- a/overviewer_core/world.py
+++ b/overviewer_core/world.py
@@ -138,7 +138,7 @@ class World(object):
             mcrs = filter(lambda x: x.endswith(".mcr"), files)
             if mcrs:
                 # construct a regionset object for this
-                rset = RegionSet(self, root)
+                rset = RegionSet(root)
                 if root == os.path.join(self.worlddir, "region"):
                     self.regionsets.insert(0, rset)
                 else: