I know it may seem arrogant to quote yourself but these are very important points that don't appear to have been considered.
HeavenShallBurn said:
This tactic is ultimately flawed.
To reiterate it seems almost everyone is either assuming a stance of either purely local defense around the castle itself or defense through intercepting fist techniques against the advancing army. These are only a tiny portion of the strategic and tactical options available to the wizards who are all mid-high level and can be assumed to be extremely intelligent.
They will not focus solely on the obvious and ignore these other options.
The enemy has a much faster decision-loop and higher mobility.
This is the most crucial advantage they possess over the advancing force. Even assuming an ideal TOC and command structure commanding 10,000 soldiers as a single operation is extremely difficult even in relatively close quarters read some accounts of civil war battles for an idea what the unwieldiness is like. Even at optimum the opfor will react and plan at a disadvantage due simply to size.
Whereas the wizards in question are assuming a flattened average 20 spellcasters of 11th level. There are only a few of them and they all presumably have access to basic communication spells that effectively mimic the abilities of modern communication devices. They are small in number with massive individual resources and extremely high intelligence. Operating as a small dispersed unit using the advantage of their command of magic this force modify and implement plans almost instantly.
They will wait until you are farther from home than destination then destroy your home behind you forcing you to turn back on the defensive yourself. When you do they will simply avoid your army. With D&D and its leveled nature as a basis the nation has too much to defend and cannot sufficiently threaten the high-level threat.
This is only one of a myriad possible strategies available to the wizards, the key is the flexibility of response their individual power and small numbers give them. To reiterate the castle is not a static defensive position, not even a base, it's a LURE.
Any defensive position may be reduced given the proper resources and time. Especially with the resources available in a fantasy setting. The purpose of the castle and its defenses is that gathering the resources required to bring offensive action against it will be a visible effort that cannot be hidden from an alert high-level wizard looking for signs of enemy activity. By focusing attention on the means to attack them a hostile reveals his intentions to watching eyes.
The wizards themselves are a highly mobile threat. With the time lag even assuming they only have that month to prepare all non-expendable resources can be distributed across a range beyond the ability to directly assault. Now that castle is nothing more than a means of bleeding resources from a hostile force. The question is not how they will stop the army, by now they don't particularly care about the army, it's a non critical target. But the enemy has now expended huge resources in offense at the expense of maintaining a defensive force. Every soldier marching out to attack is no longer available for defense. Now with their own resources well distributed and the castle to serve as a lure for the marching army and its support forces the wizards will themselves take the offensive against the heavily committed enemy heartland.
While your army is marching toward a lure they will blight your crops, spread disease. Attack high profile targets for shock effect. In short they will ruin the nation sending the force behind it both cutting off future support of the mobile force and removing the imminent threat. All the time using their magic-assisted mobility and firepower to rapidly bounce around causing chaos, panic, and disorder on a widespread basis then smashing valuable targets using the disruption caused by earlier strikes as a force multiplier.
After having destroyed the offensive capability of the hostile power behind the attack the wizards will probably only harass the enemy columns sparingly. Why? Because they WANT the enemy to spend lives like water assaulting a fixed defense on negligible value to them. They will allow the enemy to successfully destroy or take the castle after inflicting the highest practical causalties for the minimum acceptable loss.
Now the enemy will eventually have to make a choice. Do they attempt to occupy the land surrounding the castle or do they return home. If they occupy the lands surrounding the castle their name is meat. Cut off from support from home(which is now largely destroyed behind them) they will be ground down in a magic driven insurgency, a far more troublesome phenomena than the mundane kind. Assuming they return home harassment will likely be the minimum necessary to avoid creating suspicion in command circles (and with their resources they will likely have a way to know what is being said in those circles that doesn't involve scrying). But the army will find its home in tatters behind it and be forced to dispurse in order to put the kingdom back into order. At which point it once again becomes an insurgency but in this case an offensive rather than primarily reactive insurgency.
Even assuming a seige is carried out and is succesful in destroying the castle and slaughtering its inhabitants those will be the expendable inhabitants and resources. Meanwhile in the process the high-level wizards will inflict non-sustainable casualties that will cripple your future ability to defend yourself. In a short time the nation that tries this tactic will have ceased to exist.
In short any conventional threat of this sort is just plain ed. It's like the D&D version of Napoleon's Russian Invasion.