First off, let me note that a system of flat increments, such as
purchasing 10 units of a particular type all of which cost the same
amount, can exist in a balanced (or, at least, game mechanically
purposeful) way with an incremental system, such as the increased
cost per level of fortification. The issue here is not "balance" per
se. It`s reflecting a particular dynamic relationship. The flat
cost is to reflect that each unit is of the same type, equipped the
same way, etc. The incrementally increasing cost reflects that one
gains little benefit from constructing a second barbican in the
middle of the road after one has already constructed the first. A
more elaborate and extensive system is required to gain greater
defensive power. Armor that is twice as thick is not necessarily
twice as strong. The chinks of the armor remain the places of
vulnerability and the process of making armor with fewer chinks or
making those areas less vulnerable is more difficult than simply
buying a second sword.

"Balance" is a somewhat vaguely defined term. It seems to mean
something different to various people. Sometimes it means something
different to the same people based on the context. Sometimes people
seem to mean that the situation is a "fair match" with about a 50/50
shot of either side winning, while at other times people seem to mean
parity with other rules. Neither of those things are really balance,
though. Balance really means that game mechanically things will work
out in a logical and systematic way with costs that are commensurate
with the ability gained.

In this particular case, there`s nothing inherently unbalanced
against a flat cost "offensive" vs an incremental cost
"defense." What matters is what one gets for that cost. The 10
units of soldiers never become more effective as an offense. That
is, their attack values don`t increase based upon their
numbers. Their values are as flat as their costs. If the defense
values for fortification and the use of those values has more
application then an incremental cost can be balanced. One just has
to be sure that what one gets for the incrementally increased cost
stacks or has an impact that is increasingly broad in order to
justify the cost increase, while flat costs have stats that remain flat.

Also, don`t BR units exist in their own cumulatively increasing cost
system ranging from levies to heavy cavalry? Shouldn`t that be
factored into the assessment of flat vs incrementally increasing costs?

Gary