Ike said:

When one can start building level 5 fortifications, how many wall slots should one give over to towers, balancing protection against cost?

Excepting players who are "offence only" and don't care about raids on their cities (and often invest little to nothing in fortifications), I know of no one that has not opted for building 20 towers (which is the maximum no. of towers allowed). But Im sure someone will contradict me here :-).

Cheers
P:

PS: But that does not mean you should spend all your hard-earned cash on towers exclusively, IMO though I'd leave slots in walls that can be filled with towers when you're flush with Drachmas ... whenever that might be :-).

Me one i'm building in time ...when are offers up and have drachmas to spend ... they are useful on attacks for pvp points  ...also from what i read they also give a little bonus to the army who defends  ( it's possible to be wrong here )

Zsolt Antal said:

Me one i'm building in time ...when are offers up and have drachmas to spend ... they are useful on attacks for pvp points  ...also from what i read they also give a little bonus to the army who defends  ( it's possible to be wrong here )

P .. you are sure that 20 it's the number maximum of towers ??  I think i saw a lot more at some guys :)

Twenty is the maximum, report any players with more than that to the better-business-bureau ;-). In regard to fortification defence bonus, see this post.

Cheers
P.