Let`s say you know where the enemy is going to come for you, and there`s still a little time for preparations left....
If, for example I know I`ll have to fight a strong nation one day (e.g. I`d loose the space race otherwise) I`ve found it wise to get them into a war with a neigbour of theirs, then go in with a settler and grab a tiny bit on land freed up. Just one city, preferably on a Luxury (found the town right on top of it, not next to!)
it won`t produce ****, and you`ll have to rush culture like crazy, but if you then surround it with a ring of fortresses like this:
x= fortress, t = town, _ = free land
_____
_xxx_
_xtx_
_x_x_
_xx_x
important Note: there`s still a route for enemy troops to reach the city!inportant!
now the enemy will one day come ofr you - and the cannons (protected by two Infantry/Riflemen each) in all the forts will cut them down to 1 HP. Then, sweep out the Cav/Tanks/Infantry from the forts (don`t keep them all in the city or the enemy will ignore it as too strong!) and wipe the wounded enemy out! Gets you lots of promotions and takes the entire enemy offensive arm out in 1 turn!
This cannot be done without the fortresses - period!
if anyone wants I can hunt for a savegame with a town like that. Just PM me...