Spammurabi
Chieftain
This may work. The only thing I think of is that cannons were really the end of castles as fortifications (even though cannons and gunpowder just necessitated another evolution in fortification design) So maybe its reasonable; maybe Steel is a more reasonable obsolescence for castles. But simply pushing it back is reasonable.
Well, gunpowder wasn't necessarily the end of castles. Walled cities, yes, but castles in the form of forts were in existence even into WWII. Washington D.C. was surrounded by forts in the American Civil War, Paris had forts (concentrated on the North and East sides) during WWII, and Eben-Emael in Belgium was important (although the Germans captured it using hang-gliders).
Walls and fortifications were still used in the American Revolutionary war, for example the British fortifications around Boston.
So delaying the obsolescence of Walls and Castles can be argued to work from both a realism and gameplay standpoint.
I suspect, however, that an appropriate obsolescence tech may be easier to find in the Beyond the Sword expansion, where there should be greater stratification between the technologies between the Napoleonic Era and WWII.