such as resource tiles or old foundations (i captured a territory), and protectorate city tiles
#Cannot attack units on specific tiles
royal meteor
timid sigil
Also friendly camps
hushed wraith
you also can't recapture your outpost if your unit is standing right on top of it.
karmic swallow
If you have open borders but no nonaggression agreement the other player's armies can hide in your cities to escape being attacked
It's actually pretty funny