Thanks - good input. The fort is near a river and overlooks a large lake, so it does have 3 wells for water. The fort the PCs have is kind of in a no man's land between several kingdoms. It was left over from a long ago era when the bandit king took it over and built the wooden walls there. So, politics will be in the background for the near term.
So it's a completely flat plain with a lake as one of its borders, and a river to the other side? What's the distance on that river?
Food concerns can be met through cultivation of rice (if the temperatures are right) and irrigation... But I find it difficult to believe (unless this thing was built with a LARGE amount of cash) that we're looking at a large distance between it and any other kingdom. The costs alone in stone without native quarries is going to push this thing into the stratosphere. It would be a white elephant castle.
Slainte,
-Loonook.
EDIT: Saw your posts above... Farms are going to equate to people being about, unless said farms grow durable goods crops for your neighboring locales. No actual produce such as vegetables are making it to market beyond subsistence, but you could have grains, tobacco, rice, dye plants like indigo (again, based on temperature), and other resources.
A merchant traveling through to an out-of-the-way castle is going to be someone hired in the city for the trek. This is bandit country, and you have no real impetus for merchants to be going within a day's ride of this place. Decent sized keelboats or large rafts could carry thousands of pounds worth of supplies but most merchants would want you to float the cost of the raft's creation as it will more than likely be a dicey proposition to get the thing back up river.
Figure the cost of transferring materials by raft at 1 sp/500 lbs/mile away from your current location (cost of hiring a couple of deckhands to maneuver the thing, plus danger of losing the whole cargo, plus coverage of any incidence) plus materials cost to create a raft that will float. For the castle to receive shipment from a farming community 1 week away by land (that is going to be 140 miles on a conservative estimate)... It will take much longer than that (probably about a month) to get a raft down-river, and its max payload (at half speed) of 1 ton, you're looking at 56 gp. The raft is useless for anything but scrap, while a well-built raft adds an additional 100 GP (but is usable if you can get it back upriver, and halves your overall costs).
Now a barge, which you may be able to dicker down substantially, will require teams of horses, 40 men to the oars to prevent becalming, unload the materials, and generally handle cargo, and a boatswain as a skilled laborer. The barge, if loaded full at 50 tons, will require 4 horses to pull the thing at 5 ft/round, or around 4 miles/day in a 0 mph river. If there's a current that's great, but you're also going to have to battle the upcurrent, and hope the thing is pliable. For a full load you will need a team of horses 5x larger than that, but they will be able to move your materials at a working speed of 20 miles/day back to your castle. A barge runs around 6000 GP, with an outlay of 4000 GP for your 5 teams of 4, and 800 for the chain, tackle, and saddle required to secure the team (just using simple block and tackle/saddles/chain costs here).
Now the delicious opportunities to pull 50 tons of materials is going to help out a lot, and that outlay can be somewhat offset by using your warhorses to pull the barge... But you're going to leave yourself short in the heavy horse department.
But, this doesn't take into account your PCs. Want to make some money? Have the PCs go along, trading, offering spellcasting assistance to local businesses and farmers, having the men doing odd jobs and Profession/Perform checks while on the boat. This wasn't uncommon (without the spellcasting of course

) into the 20th century, and may provide for some little adventure opportunities along the way.
Once trade has been established by your travels up and down the rivers looking for materials you could hire out boatmen to maintain the more skilled travel, and begin to purchase horses to pull your barge. Perhaps you can send smaller keelboats (20 Ton capacity but under oar and sail) in lieu of the barge to begin, with the PCs being the main individuals on the ship and working towards a barge. The Keelboat runs around 3000 GP, and could easily be dry-docked in some hidden locale or on the inevitable dock that is going to be located here near your lake. Commandeering any boats on that lake and putting able-bodied semiskilled and unskilleds to the task will really help you, and the NPC groups can be randoml determined to succeed or fail as they sail.
Slainte,
-Loonook.