The spell and the actual wall are the same thing - they don't stack.
I find walls all-but-useless as they currently are. In half the time it takes to build them, you can build an obelisk, so for most cities you would have expanded culture and 20% defence before the time it would take you to build walls (to me, extended cultural borders are better for defending yourself than the 20% anyway). Plus, starting on your culture early means you'll get to that not-so-hard-to-reach +40% culture defence bonus - which of course makes the time you spent building walls a 99% waste.
As mentioned, you need walls for castles. However, castles come much later and, truthfully, I've never used castles to defend a city anyway. Building castles in new border-towns is too much time and building them in your strong cities is pointless since they're already at or near 80% culture defence.
The only time I've ever built castles (and, hence, walls) is for the special units they provide. It's not a defencive consideration at all.
I would like to see walls and even castles get more importance in defence. For instance, defencive bonuses from the two sources could compliment each other. I don't mean stack, but when bombarded the complimentary bonuses would each have to be reduced, therefor halving the effectiveness of the siege attack. For example, a city with culture +20 and walls (+25) gets bombarded by a catapult (10% bombard if I remember correctly). The first 5% disappears as normal, but the next 5% from the catapult is split between the two, bringing the city defence down to 17.5%. Next bombard by that catapult will be completely split, so only brings the defence to 12.5%.
Anyway, enough of my ragging on this issue.
- Niilo