Before I start, this is NOT a balance suggestion.
Usually I barely use towers, sometimes to secure a resource spot against an archer ambush or not at all. Same goes for stone walls. The only real use i have for them is usually in late game to secure vills from hussar raids and even that is just situational. I also barely see stone walls and towers in tournaments. The reason is quite obvious. Stone is so rare, that even gold feels cheap compared to it, so it is nearly completely used for town centers and castles. On the other hand, palisade walls se a heavy use and walling with buildings in addition is something like the meta in ranked games. To me, this concept always feels a bit strange. Medivial battels in open fields were rare and laying siege on castles and cities was something like a standard. Having barely stone walls but walling with wooden houses. palisades and even marketplaces is efficient in the game, but in a way absurd. Concerning that, I ask myself, if the whole stone concept in the game has not turned everything to a point, that was never intended (I doubt that the developers in vanilla thought, that walling will be done with with everything but with stone walls).
After all, I think, that stone and stone costs might need some rework in general. Besides that, the Bulgarians Krepost as something like a castle light fits so well in the game, that there is maybe a demand for more tactics with defensive structures.
Things that come in my mind are:
No stone costs for town centers but with other extra costs (maybe more wood and/or some gold), to free the starting pile of stone for some defensive structures.
A bit more stone on the maps and a slightly higher stone mining rate, but an increase of stone costs for castles (effort for castles should not change much, but costs for other dfensive structures should be more attractive that way)
Make it possible for villagers to mine a part of the used stone from a castle or tower, once it is destroyed (stones were often used from old structures)
general slight reduction of costs for stone walls
more light castle options like the krepost
less hitpoints for buildings that cost no stone, to make walling with them less effective.
Beware, that all theese things are just a theory and not a request to immediatly implement/change and nothing here is for reasons of ballance.