As for the necessity, one may sum it up like this.  Magic, in most

fantasy 

novels, is not a mass-produced affair.  The appropiate analogy in

"reality" 

may be the transition from highly specialized bands of mercenaries

serving as 

the military might of a single feudal lord to the foot soldiers and

standing 

armies of viable governments.  A wizard is a highly trained,

incredibly 

specialized (or, more likely, an incredibly special) individual; the

catapult 

technology, on the other hand, can be taught with relative ease

(after one 

has achieved a basic mastery over the even easier concepts of

preceding 

simple machines), and the resources to produce them are widely

available.  

The result is a cost-effective alternative.

	Exactly. For magic to have existed at one time, and not now a days one
of two things had to happen:

	1.) The knowledge was completely lost or was taken away/forbidden.

	2.) Technology was easier.  Magic simply became obsolete.

Adam Chris Leigh
Omicron@sprynet.com