Thanks. I did not know all of those facts.



For canal use you should note that 2 forts together can form a canal, but beyond that will not work. Thus if you have a 2-tile thick land mass, you can create your own exclusive panama-style shortcut across the landmass. There are special cases where you can pass thicker areas, for example if there is a fresh water lake within 2 tiles of the ocean, and within 2 tiles of the other side.



I'd also note that forts can be destroyed by spies and air units. The cost for spies seem to be comparable to other improvements that take the same number of turns to build. Air units, however, have a much harder time destroying forts than regular improvements and even bombers will usually fail several times before they destroy the fort.



I do know that land units will stay put if the fort is destroyed. Air units are transferred to the nearest city or fort within your borders that has space available. If there is no space available anywhere, the air units are destroyed. Naval units are expelled to the water.



About the pre-building. Sometimes with spies or planes the enemy will try to destroy your strategic resource improvements. If this happens to be your only oil source, it can halt progress on important units. If you have an oil source within your big fat cross, build the well but then prebuild a fort so that if the well is destroyed you can immediately get a worker there and complete the prebuilt fort so that there is no lapse in access. For oil outside of a big fat cross, you can prebuild the well over the fort. In both cases, be careful not to actually complete the prebuild though. Also a good idea to keep a spy and interception-promoted SAM on those tiles.

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