My bad guys have holed themselves up for a couple months. Given little preexisting fortification, what resources are there for for building mechanical and magical defenses? Is there something equivalent to 3e's Mage's Magnificent Mansion? How about Wall of Stone? I don't even know where to begin with magical defenses, but presumably those exist as well.
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The fastest way is to ritually create a stronghold with a level 20 ritual. Assuming you don't have the levels for that,
The article notes:
The article provides the following ritual:
Lower levels have a much more difficult time of it, as the idea of manipulating terrain is something best handled in a skill-challenge rather than mechanically.
My recommendation, if the players don't want a stronghold, is to turn it into an extended skill challenge, with each success in arcana, dun geoneering, history, nature, thievery, etc.. providing some interesting feature which they can blow up/use later in the expected epic defense battle. Every failure wastes time or resources.
Aside from magical powers of any kind there is a historical model of fortification involving simple manual labor that may useful to you.
Roman Legions could construct a "castra", a fortified camp in as little as a few hours even while under attack. These came in many different variations depending on how they were used and how long they would be used. Wikipedia roman legion castra
If your bad guys have any knowledge or experience with fighting and defending, (if not they should) then in a couple months with just manual labor they could build a very respectable fortification with out any magic used at all. Plenty of stakes in the ground, pit falls, walls, gates, and even a few towers if that would be appropriate. They could even have stock piled munitions and fighting supplies at key points in the defenses. All these would be tailored to how they intend to fight and defend. Bad news for an attacking party especially if the bad guys know how the party fights. They would prepare for that.
Since it sounds like you are the DM, don't worry too much about "magic building techniques" and just build the fortification. The party isn't going to be analyzing how it was built, but more on can they survive breaching it.
This is a resource for DnD 3.x, but you may find good spells to turn into rituals for use in 4e. I ran a campaign with it, and it did large-scale battles really well. Eden Odyssey's Fields of Blood: The book of war The Q&A site for it is here.
It has rules for building fortifications, cities, and the like. One of the spells used was a spell that made a 5ft bump of earth, with a 5ft ditch behind it. It was a low-level spell, and the Druid used it a lot. He could also move a ton (many tons, actually) with a stone-shape spell and all but put the construction industry out of business! Each casting of that 3rd level spell could produce 100 squares of 2x5x5ft stone. (20cuft at level 10) See this posting for an answer on translation stone-shape to 4e.
My actual suggestion for the badguys using 4E as-is:
And besides, you're the GM. Do whatever makes the PC's squeal. :D