Okay, let me preface this by saying I'm not even sure its a good idea or that I would want it. It may have already been talked about.
Static Province defenses (in addition to troops)
PD 10 1 flimsy tower
1 xbow (AP) bolt/2 turns, 50 hp, 10 prot
PD 20 2 flimsy tower
1 xbow (AP) bolt/2 turns, 50 hp, 10 prot
PD 30 1 Medium tower
1 flaming xbow bolt/2 turns + 6 AP damage, 100 hp, 12 prot
all the way up to the max (125?) and maybe a shadow blast, with various spells in between.
etc, etc probably placed in front or mid but with 50-100 range.
Probably doable by modding a unit to be immobile and adding it to PD at the various stages. Pretty hefty HP/protection so it would generally be near the last to die. Maybe make the 10/20 PD versions smaller hp but increase greatly once past 20.
125 PD
2 Strong Shadow towers
1 shadow blast/turn, 300 hp, 20-24 prot, add a bit of defense or make lucky, ethereal or something.
Other ideas, harder to implement:
Archer cover wall +10-20 shield prot
Raised archer/mage tower +20-40 range
Various army buffers (allied or both)
Various static battlefield spells
If it proved to be too much, maybe just make them a site (not sure if that would work). Uncommon to rare, adds defensive buildings to PD.
random thought
