There should always be more things available than you can afford to buy...
Here are some ideas I had:
1) Implement the design-a-castle extras for Construction (add crenellations,
moat, porticullis, etc), but make them cost a wad of money. These extras
are mostly for show, but would add a small amount to the defensive
ability of the castle.
2) Sell some useful items such as pikes and longswords at market, but
make them expensive.
3) Add another group of NPC's: Mercenary units. These would be groups
of soldiers which would move around on their own, but which could be
hired by players. These units would stack with players who paid them.
They would probably have 50-100 soldiers, and charge a minimum fee 10
times their usual maintenance cost.
-- Rich Skrenta <firstname.lastname@example.org>