The walls mod has been updated to patch 1.05
Grab
http://users.tpg.com.au/jfwfreo/walls.zip
Unzip it into the Lang-english\1.5 folder under your C&C3 folder. (or if you have another language version of C&C3, the Lang-whatever\1.5 folder, wherever it is that patch5.big is located)
Then you will have walls again. No changes made to the walls as they were in the previous mod (which means AI probably still wont attack them or build them)