What I was thinking about was how difficult will be for a player that has no Heal spell ( both Good or Evil ) to assault a fully upgraded castle ? We saw that certain factions have access to heal via heroes or special units but, wouldn't it be still a problem if the defender ( Dwarves, Rivendell, Angmar, Gondor for example ) barricades himself and while using all possibilities drains a massive army that has a lot of fighting potential but no supplies ? I also consider that this should not result into losing the whole offensive that will cost not only force but more important, terrain control and resources.
On a side note though, certain maps that are usually narrowed ( Stonewain, Nanduhirion, Last Bridge ( new one for 4.5 ) and many more ) how will they function now when all settlement buildings don't have defensive towers ? Will the players defend those and return asap to their bases for heal until some takes the initiative for a full scale assault ?