Makes me wish we had the terraforming tools that Alpha Centauri had or at least be able to make some improvements not overwrite others ie forts shouldn't replace farms etc. Would make forts more useful as right now they are really only substandard for defending bottlenecks and encampments do a better job of it.
Digression on 'Forts':
1. The stand-alone Fort for most of history was a place where defending forces were housed as well as where they defended - like Roman Legionary Forts, Medieval Castles, or 'strategic' post-Renaissance fortifications like Schweidnitz built by Frederick of Prussia.
2. Forts defended something. - And in many cases, provided refuge for the civilians farming or working the land around them, as in the Celtic Oppida (at least in southern Britannia) or the Medieval castle.
Therefore, Forts should be like Roads, an overlay on other Improvements already in the tile, but in addition to their Defensive Function, they also make it impossible for an enemy to Raze or Pillage anything in the Tile. Forts should also come about two Eras earlier, possibly at Engineering along with Ancient Walls. Then, with Tech: Castles you could build the Castle - improved Fort, with possibly a +6 instead of +4 Defense and, in connection with Civic: Fuedalism, each Castle becomes a Fief. A Fief automatically creates a Free Knight whenever your Civ enters a war with another Civ. This Knight requires no maintenance, BUT each Fief reduces Loyalty in the local area, because the locals are transferring their Loyalty from the Central Government to the local Lord/Knight. The Fief also removes all the yields from the tile that includes the Castle - the knight is, basically, paying for himself by 'levying' the local peasants. Build too many Castles, and you will more or less strangle your own cities! With the Renaissance Technology of Metal Casting (Bombards) the Castle becomes obsolete as a fortification and 'seat' of a Knight, but it can be 'refurbished' as one of several things:
1. a Renaissance 'Fort', or Fortress in the classic rampart and ravelin Star Fort design, which provides +10 Defense to an occupying unit. This will require 2 Builder Charges if built on a bare tile, or 1 if converting a Castle.
2. a Chateau for France - many of the Renaissance Chateaus, like the Rezidences in Germany or the 'Manor Houses' in England, where converted from older castles.
3. Leave them alone, and along about Civic: Natural History they become Romantic Ruins, tourist attractions that provide Tourism, Culture, and some Gold.
If the game had a more dynamic Settlement mechanic, any Fort would also provide a 'nucleus' for a civilian hamlet, village, town, and eventually City to grow up around it: as in, for example, Mainz, Germany around the huge (2 Legions) Roman Fort there.