As per my knowledge, HA is a must, however, Forts do act as HA, so maybe that is what it is. This was discussed in the BETA phase and it was agreed that HA should be a necessity.
The reason- you want your guns to throw shells far into enemy territory such that your own troops are not part of "Collateral damage".
a.k.a. hara-kiri is to be avoided.
Rascals, would you live forever? - Frederick the Great.