No, I'm not talking about the points you need to cover in your next
History exam, I'm talking about the late Roman/East Roman/"Byzantine"
system of military reserves.

I've been long trying to come up with a way to convert this to game use.
And, in Birthright, army maintinance can be a crushing burden, and many if
not most realms have trouble dealing with it.

The "Theme" system was a system of military reserves, kind of like the
Israeli system (or the US National Guard, except the Thematic troops saw
combat a lot more frequently, and thus were more "combat ready").
These slags were equipt by the government, given a plot of land (much like
Knights in the west), and semi-regular pay. For the most part Thematic
cavalry would be very similar to Khinasi Medium Cavalry in ability (the
East Roman "regular" armies of, say, the late 10th century, the Tagmata &
Imperial Guard units, would closely resemble Elven Cavalry, Elven Knights,
and Imperial Legion units; at that time they were *damned* good, "best in
the world" kind of thing).
This is the closest I can come to a conversion, and creative rulers with a
lack of funds might want to consider such a thing:

Unit Muster Cost: Identical to the real unit, x2 (to aquire a land holding
for the troops; meaning a potentially large initial investment, but see
below).

Maintinance: in peace time, the ruler only needs to pay Maint Costs for the
unit once a year (I.E. 1/4 Turns). when the units are mobilized, normal
maintinance costs must be paid each Turn.

Mobilization: takes a week to mobilize the unit(s) in whatever province
they are located in. Units cannot be "destroyed" by an enemy if they are
not mobilized, but they cannot be mobilized in an occupied province (they
stay on the farm; units *can* be eliminated by Pillaging, however: see
below). If the units are mustered already and elsewhere, they are not
eliminated, but they cannot return to the Province if all units survive the
war (either new lands must be found for them, or they must be disbanded, or
they are converted to regular troops).

Limits: 1 unit per level of the Province. If an enemy Pillages the
province, they get to chose which (if any) units are destroyed. A Province
may not loose a unit, however, if a full compliment isn't present (I.E. if
a Province (4) has 3 units of Thematic troops, the 1st time the Province is
Pillaged, no unit is destroyed).

Replacement: If a unit is destroyed, it can be replaced by spending the
normal Muster cost for the replacement unit (it is assumed they aquire the
lands of the eliminated unit, being sons or other relatives of the slain
troops, or just a re-muster of scattered survivors, etc), not twice the
cost (I.E. the initial x2 Investment is a one time expense). The replacing
unit does *not* have to be identical to the eliminated unit if its muster
cost is the same or lower than that unit. (I.E. you can replace a destroyed
Elite Infantry unit with a regular Infantry unit and accrue no extra cost).
Replacing them with a more expensive unit requires you to pay the
difference in a bonus.
I.E. a unit of Thematic Cavalry is destroyed in battle, and the ruler
decides to muster a new Thematic unit to replace them. After much
consideration, he decides to upgrade the unit to a Knights-equivilent.
Cost: 8 GB [6 GB to muster the knights, + 6-4 GB, the difference between
the cost of Knights and Cavalry.]

Standard levys are absolutely worthless, so this system is somewhat of a
compromise between Levys and a large standing army.
Oh, and unlike levys, if slain outside the province, the actual province
level is unaffected. But the # of potential units are limited (I.E. a
Province (4) could theoretically support 10 units of Levies, but only 4
units of Thematic troops could be based there). The other advantage over
levies is that the Tax level of the Province is unaffected by mustering
Thematic troops. Levies are much cheaper over all, but you get what you pay
for. . . . . .