I had the same problem... my only source of iron was between two of my cities which were just far enough apart that after a couple of cultural expansions, there was still a narrow strip of 'unclaimed' land between them, containing my iron and therefore my colony. Because this was on a long, thin peninsula, I didn't want to drop a city in there (in retrospect, I should have).
At any rate, one of the AI players sailed a galley past, and then must've traded world maps with the other guys, because pretty soon, they're all sailing galleys to that spot. There were two squares on the coast where a unit could come ashore that were not inside my national borders... the colony square and another. Each time the 'invasion' was the same, regardless of civ, and regardless of whether we were at war. The galley sails up, and a spearman lands in the non-colony square (maybe you cant come ashore onto a colony, considered an amphibious assault?) and then a settler would come ashore in the same square.
If I was at war with the civ, I would march out an Immortals unit (playing as Persia) from the closest city and kill them before they could settle a city next turn. If not at war, I would bully them in diplomacy, trying to goad them into war (never worked, I always declared war) then kill the invaders (earning two workers each time from the captured settler) and then sue for peace. I had no choice... this was my only iron source, and no Iron = no Immortals, and no Immortals = no crushing the worthless Aztec Scum! Mwuhahahaha......!
Sorry. At any rate, I quickly learned to defend both the colony square and the other square, to totally prevent landings from sea. (Being on a long penisula, I had long ago sealed my borders to the south from the WORTHLESS AZTEC SCU....
Ahem, yes. As a general principal, in future games, any single-source strategic resource i have that is not well within my borders and highly defensible will not only have a defender in the colony square, but also a strong attack/defense unit (like immortal in ancient times) or combined arms pair (archer/spearman) on hand to chase away WORTHLESS AZTE.., i mean, potential squatters.