To understand why we have to use rams when we attack will take the following example:

- the attacker have 20.000 Imperians;

- the defender have 20.000 pretorians and roman wall at level 20;

- the players have the same populations (they do not have bonus for small population)+(no bonus from any hero).

The result is that the player who attack do not destroy neither 50% from defense:

Attacker          
  Troops      0 0 20000 0 0 0 0 0 0 0
  Casualties 0 0 20000 0 0 0 0 0 0 0
Defender  
  Troops      0 20000 0 0 0 0 0 0 0 0
  Casualties 0  9830  0 0 0 0 0 0 0 0

We will add 200 rams to the attacker and we will see a grow with 30% at the defense destroyed:

Attacker          
  Troops      0 0 20000 0 0 0 200 0 0 0
  Casualties 0 0 20000 0 0 0 200 0 0 0
Defender          
  Troops      0 20000 0 0 0 0 0 0 0 0
  Casualties 0 12685 0 0 0 0 0 0 0 0

We will add another 300 rams and we see another grow with 80% comparatively with first attack:
Attacker          
  Troops      0 0 20000 0 0 0 500 0 0 0
  Casualties 0 0 20000 0 0 0 500 0 0 0
Defender          
  Troops      0 20000 0 0 0 0 0 0 0 0
  Casualties 0 18615 0 0 0 0 0 0 0 0

We will add another 200 rams to the attacker:

Attacker          
  Troops      0 0 20000 0 0 0 700 0 0 0
  Casualties 0 0 18816 0 0 0 659 0 0 0
Defender          
  Troops      0 20000 0 0 0 0 0 0 0 0
  Casualties 0 20000 0 0 0 0 0 0 0 0

We observe that the attacker will destroy all army and will remain with army if he attacks with only 700 rams who are be ready in only 5 days. A grow with over 100% at efficiency.

You will say that 700 rams eat a lot of crop and there could be put other army in his place.
We will make the next simulation : we change the 700 rams (2100 crop consuming) with 2100 Imperian (2100 crop consuming).
Attacker          
Troops      0 0 22100 0 0 0 0 0 0 0
Casualties 0 0 22100 0 0 0 0 0 0 0
Defender          
Troops      0 20000 0 0 0 0 0 0 0 0
Casualties 0 11284 0 0 0 0 0 0 0 0

The result is very weak comparatively with what we win if we use rams.