Indeed. Investing a fortification merely incurs negative levels, it doesn't damage/destroy anything. Sieges certainly do damage. Same with assaults; but in the case of an assault there is a good chance SOME of the fortification will remain. If you just siege it into oblivion nothing will be left.
The way it's supposed to be done is like this:
- Move in and neutralize
- Invest (not shoot at the castle, but conceptually including preparations for shooting/assaulting)
- Besiege (actively shooting shit at the castle)
- Once you've succeeded in knocking the castle down a few levels, plus incurred a negative level or two...
- ...assault!
- Given some luck the assault with cause reparable damage only, and the negative levels go away once you've put in a new garrison
Example:
- Ghoudaïa has a castle 6*
- KoH/IHH army finally battles through and invest the castle**
- KoH/IHH sets up shop and the dwarves start shooting rocks at the walls, sapping etc.
- Every 1 month/4 WM the attackers can try to damage a castle level***
- Every 3 months/12WM the castle takes 1 negative level****
- After half a year the castle has taken 2 points of damage and one negative level (there was a counterattack that lifted the siege one meson)
- A castle 3 is assaultable for the KoH/IHH army
- On the third attempt (3 WM of trying) one castle level is damaged, bringing it down to effective lvl 2*****
- The fourth attempt does no damage, but the siege effort does. Effective level is now 1
- On the next WM the remaining garrison tries to surrender, but KoH/IHH declines. Another assault is mounted, doing no damage to the castle, but routing the defenders. The city is put to the sword.
- The negative level goes away, giving the attackers a castle 2 with 4 damaged levels. The DM decides that 2 of those levels can be repaired for 5GBs each, one more for 10 Gbs, and the last one is destroyed.
* We'll disregard other fortifications for this example as they will only complicate stuff
** Requiring 24 units if we assume a full garrison and active castle
*** Castle garrison units can also take damage from bombardment, but this is a secondary effect
**** It's important to keep up the investment; if the enemy breaks through that turn's negative level will be lost, or worse - negative levels can recover! Units inside can also take damage from starvation
***** Defending units are also taking damage, but MUCH less than the attackers