That's not true in Heroes 3.5. The strength of the towers depends on the number of attacks on the city! This has been verified many times, if you doubt it, check it yourself.
Sorry, I don't play 3.5.
How did you check that? Do you always stay in the castle with just one hero?

Как много в жизни есть путей
Для добродетелей и мира.
Я вернулся...