Argthtjtr

I compared the early ranged units. I'll abbreviate melee strength with MS and ranged strength with RS.

Unit             MS RS

Composite bowman 7  11

Catapult         7  8



Crossbowman      13 18

Trebuchet        12 14



Cannon           14 20

You get composite bowman and catapult about the samt time in the classical era, and the crossbowman and trebuchet is available in the medieval era. Also roughly about the same time.

The composite bowman is (as far as I can see) better in all aspects compared to the catapult. Same goes for crossbowman and trebuchet. And the catapult and the trebuchet have the huge drawback that you have to set them up before firing, which means that the enemy gets the first shot. The cannon which comes in the renaissance era is slightly better than the crossbowman, but not by much.

I know that the siege units (catapult, trebuchet, cannon) gets a bonus when attacking cities, but apart from that, is there any reason to use them? The only reason I can think of is that when you upgrade a crossbowman you get a unit with less range, which is a huge drawback, while the cannon upgrades to artillery that has 3 in range.

As you have noted, the main difference between the "seige" units (Catapult, Trebuchet, Cannon) and other ranged units (Composite Bowmen, Crossbows), is that the seige units have a bonus against cities.

However, it is not a small amount - each of them has:

Bonus vs Cities (200)

As such, your table against cities will look like this:

Unit             MS RS

Composite bowman 7  11

Catapult         7  24



Crossbowman      13 18

Trebuchet        12 42



Cannon           14 60

Whether you value this benefit enough to warrant their deployment, compared with the more mobile and higher base-damage units such as Composite Bowmen - is entirely up to your strategy.

However, the Bonus Vs. City is the main advantage of these units, and should be considered the main differentiating factor between them and the other ranged units.

As you said, they get a bonus when attacking cities. This is not a small bonus- it's a 200% increase. That's 24 strength for the catapult (vs 11) and 42 (vs 18) for the trebuchet. When you're dealing with the high strength, health, and regeneration of fortified cities, you'll need that strength.

Another incentive to use siege units early is to increase their level to get the Range promotion (+1 range) as early as possible.

Early cities are realatively weak and need a few rounds to destroy a siege unit. You can attack a city, retreat, heal, attack again, etc. Later cities, in combination with occupying units, can obliterate a siege unit in one round, so this "training" is not possible anymore. If you have trained 2-3 siege units early on and keep them alive you can siege a city without it being able to shoot back. It's almost too easy from this point on. Even more so, if you also choose Logistics (1 additional attack per turn).

In summary, you build them early to use weak enemy cities or city-states to train elite siege units for later.