One legitimate argument I have about the towers is that their cost is pretty balanced with the number of units required to kill them.
Tower cost: 250 wood
Pikeman cost: 40 food and 40 wood
Therefore roughly 3-4 pikemen is the same as 1 tower. But once the towers are upgraded, they become significantly better, although the upgrades are very expensive (400wood/coin and 800wood/coin if I remember).
In reality, a tower similar to what is in the game should be much more powerful than that, but would be very expensive and difficult to build. Just based on the look of the towers, they should be able to accomodate up to 4 shooters (or 1 cannon).
I think that to benefit turtlers a little, the towers should be made more flexible. If you build a tower, that tower should be able to 'train' 3 extra shooters up to 4 total (probably at the same cost of musketeers). The same thing could be achieved by garrosoning 3 soldiers with ranged attack. And bombard towers should have some flexibility in that you can prescribe them as a 'falconet' type cannon or a 'culverin' type cannon kinda in the same manor as you can change the specialization of the factory. This kind of change should take several seconds so as to not give too much of an advantage to the turtler. I don't think it would give much of an advantage anyway because any human player would attack towers with mortars, which have a longer range. With this method you run into problems based on the fact that you may have paid for many extra shooters, but when you upgrade the towers to bombard type, you lose that investment. Maybe any extra shooters you trained become actual foot soldiers when you upgrade, I don't know.
Just a thought, because I agree that defending in this game is a little underpowered.