Adding files I forgot to commit:

alpha mask for cutting tiles (alphamask.png), and updating "built.cfg"
file to reflect the change from [terrain] element to [tile] element

data/built.cfg

@@ -9,10 +9,14 @@ C
 
 [building_rule]
 	map="
-!KC
-  !KC
+1
+  1
 C"
 
+	[tile]
+		pos=1
+		type=!KC
+	[/tile]
 	image_background="castle-wall-convex-$R"
 	rotations=ne,e,se,sw,w,nw
 [/building_rule]
@@ -22,11 +26,11 @@ C"
 C
   C
 1"
-	[terrain]
+	[tile]
 		# Using anchors for demonstrations's sake
 		pos=1
 		type=K
-	[/terrain]
+	[/tile]
 
 	image_foreground="keep-inside-$R"
 	rotations=ne,e,se,sw,w,nw
@@ -37,12 +41,12 @@ C
 !KC
 
 K"
-	[terrain]
-		# Using [terrain] element just for demonstration's sake
+	[tile]
+		# Using [tile] element just for demonstration's sake
 		x=1
 		y=0
 		type=!KC
-	[/terrain]
+	[/tile]
 	image_foreground="keep-wall-$R"
 	rotations=ne,e,se,sw,w,nw
 [/building_rule]
@@ -67,4 +71,64 @@ K"
 	rotations=ne,e,se,sw,w,nw
 [/building_rule]
 
+# Some test cases
 
+#[building_rule]
+#	map="
+#.   1
+#  .
+#.   1
+#  .
+#.   1
+#  1
+#1"
+#	[tile]
+#		pos=1
+#		type=m
+#	[/tile]
+#
+#	no_flag="built-m"
+#	set_flag="built-m"
+#	
+#	image_background="mountains-test"
+#[/building_rule]
+#
+#[building_rule]
+#	[tile]
+#		x=0
+#		y=0
+#		type=m
+#	[/tile]
+#	no_flag="built-m"
+#	set_flag="built-m"
+#	image_background="mountains-test-2"
+#[/building_rule]
+##
+
+# A stupid test case, just to demonstrate the has_flag condition
+
+#[building_rule]
+#
+#	[tile]
+#		x=0
+#		y=0
+#		type=C
+#		set_flag=moo
+#	[/tile]
+#
+#[/building_rule]
+#
+#[building_rule]
+#	[tile]
+#		x=0
+#		y=0
+#		type=C
+#	[/tile]
+#	[tile]
+#		x=0
+#		y=1
+#		type=*
+#		has_flag="moo"
+#	[/tile]
+#	image_background=schmurgle
+#[/building_rule]