Destroyers are not designed to destroy city's anyway, their main purpose is to detect and destroy enemy submarines.

Young wrote:

Destroyers are not designed to destroy city's anyway, their main purpose is to detect and destroy enemy submarines.

Quite incorrect, as destroyers existed prior to the use of Submarines in WW1 in fact, they in fact were intended for other purposes. Meant for patrols and escorting vulnerable ships by screening them against other small armed enemy craft they were upgraded to preform new tasks as the technology became available (such as torpedoes to engage larger enemy ships, anti-air guns once Aircraft became a viable threat, and of course depth charges during WW1 when they needed a simple solution to U-boat threats). At D-day when long range shelling of battleships and airstrike weren't able to take out German bunkers effectively, Destroyers closed in and provided close support for the troops by blasting the Bunkers themselves, even with lighter firepower 5in guns up close gave them far superior effect than battleships in accuracy. Destroyers and all naval armed vessels were at their core platforms for carrying artillery, which is effective versus land and sea targets. While they aren't meant to hit cities, they are good at providing close artillery support for infantry near a beach or river deep enough to permit a destroyer clearance, in the very possible event a city is on the coast a destroyer would most certainly be able to attack it.

What he meant is that if your read the description of Destroyers in Call of War, they have a specifically designed mission to protect the other Capital Ships from Submarines. You cannot use Destroyers in Call of War to attack coastal cities. There are several threads that discuss this ad nauseum, Try searching for something instead of opening a new thread and you might see all of the excellent discussion on this topic.

All ships except for the BB should get a boost against buildings and ground troops on land(keep in mind subs are boats, not ships). This is because it really doesn't matter what something in war is designed for, it is usually used for it other stuff too. Two examples are tractors and PIAT's. Tractors, while meant for farming, are used for dragging artillery around or mounting anti-tank guns and howitzers on them to turn them into a tank destroyer or a SPG. For the PIAT, it was meant as an anti-tank weapon for infantry but was often used against infantry.