I'm playing a Kingmaker campaign as a druid at the moment. We're at the point of establishing a town and starting to build a Kingdom. I'm only level 4 currently but have been looking through the spell list for interesting opportunities that can be used for kingdom building, defense and so on.
Reading through Awaken it seems you can get an awful lot for your money. If you can find yourself a Gargantuan Tree you can get yourself a 10HD, CR 9 permanent helper/guardian for the low, low price of 2000GP. You can go off adventuring safe in the knowledge that a giant, sentient Tree watches over your town. You can even technically cast it multiple times, but this means the original trees don't need to obey your commands. They may do if it's in their best interest though.
There are a few other possibilities as well:
Plant Growth probably wouldn't be very useful as each Hex covers 150 square miles and each casting covers less than a square mile (0.78 sq miles, approx). Even with 4 castings a day it'd take you a month and a half to cover a Hex. I suppose you could use higher level slots to increase coverage per day, but it's still a huge effort for minor gain, DM permitting.
I've seen a couple of suggestions on these boards about using Wall of Stone to create City Walls, Castles, Bridges and suchlike.
What other interesting and permanent or semi-permanent uses of spells are there out there for Kingdom building?