I'm pretty sure we already use this, but for the sake of completeness let's start with
Celestial Fortress. It creates a structure similar to
Leomund's Secure Shelter, but in the form of a flat roofed fort themed after the caster's deity. The inside is also effected with consecrate and circle against evil, which is the real selling point. Its too small to be useful alone, but as guard posts and impromptu defensive positions they're good. It does require a 1d2 strength damage to cast, but if we make it as an item we can have different casters take turns replacing it. At minimum CL it'll last for 16 hours, which is plenty of time to establish a replacement rotation to minimize the number of items needed.
Next up is another LSS variant,
Leomund's Billet. This is a level 5 spell, but it provides basic housing and food for 40 people and lasts 2 hours/ CL. Paying the upfront cost in items to shorten up our logistics network is well worth the effort in my mind. Tents and create food spells are nice, but the added security of a shelter you need a minimum of siege weaponry to knock down is considerable.
These two would form the core of a camp, providing food, shelter, and basic security checkpoints at important locations. To supplement that, we'd want control the environment more, and have some way of making entry more difficult.
The start for that would be
Battlefield Fortification which lets the caster make trenches and berms. They're 5 ft long/CL and 20 ft wide, which is good for camp defenses and trench warfare.
Ideally we'd set up somewhere with useful terrain features, then supplement them with trenches, stone walls, and CF choke points.
I'm still working on the stone wall stuff; the simple answer is using wall of stone, but that's expensive with the other buffs I want to use.
If we can get any kind of stone structure up, we can use
Stone metamorphosis to turn it into something highly durable, then hit it with
Earthfast to double it's HP and give it a hardness of 10 if it didn't have at least that much already.
Sure you can get in with a few different forms of magic, but most of the ones that work for groups aren't subtle.
To top it off, we add some
Aerial Alarm items ( which I think we already use) to make stealth attacks from above harder.
I'd also like to get some mages capable of using
Legion's Undeniable Gravity if we can. It's a long range spell that hits a 20ft burst and forces everything inside to make a will save or start landing. Here's the base
spell for it. Maybe we could use some
Spell Latices to make it available?
