How do I deal with the edge tile?
How do I build/dig on the edge tile of a map? My dwarves are walking around the edge of a moat; I assume invaders will too.
You cannot dig or build on the edge tile, to prevent you from completely cutting yourself off from the world (for good or ill). Most people simply dig a most 4-10 tiles in from the edge of the map.
You can't. You'll have to find another way to block off access. Sounds like you might be able to build a wall up to your moat; get creative!