You can also use a forest as a "super fortress" If you have a tile you have to keep - say a choke point - a simple fortress might not be enough. So once the fortress is built, plant a forest too. The unit will get both defensive bonuses. Personally, I forest the unworkable tiles between cities - often the AI city placement leaves gaps that cannot be worked, but not so many to be worth a new city. So I forest the area between the cities, so that I know not to bother railroading that area. Also, although a forest is the same no of shields as a mined grassland with rails, it's one less food. If I'm trying to balance out the food and get an odd food left over, I might plant a forest to lose the odd food. (saves the tedious growth and starvation cycle).