Keeping a contiguous front line is very important – otherwise enemy units can crawl around back there and steal areas.
You can be daring with the leader just before you change into the next phase as you'll get a new one right away anyway.
Forts are good blockers in the Kingdom stage.
Ships are probably the best way for a barbarian to fight an Empire – compare costs.
Holding the mountains is valuable – holding the forests not especially so unless opponent primarily mounted, e.g. Huns.
A single feet in the Channel is invaluable in preventing crossings.