Attack all the Spanish towns?

Your reputation needs to be low enough in order to attack cities.
The easiest way would be to sink some (maybe a dozen) spanish ships and hit space near spanish cities (= fire on them) until the cities start firing back at you.
At that point you should have enough "hate" to be given the option to either attack or sneak in upon entering the city.

Attack by land.

Maybe he is suggesting that he cannot invade at all.

You can attack only cities which have visible forts next to them.
Towns without these can never be attacked.

Towns can always be invaded. These are the ones with the large flag overhead.

Those pesky tiny things cannot. I like to call them villages, much smaller flags overhead.

If it has a governor, you can smackey it. For those things that only have a Mayor, they made some kind of programming decision to make it not-possible.

For those things that only have a Mayor, they made some kind of programming decision to make it not-possible.

True. However you can always talk to the Mayor, invite him out for a sail, and turn him into shark-bait. Not as good as turning over the town, I admit, but it does occasionally pass the time quite nicely.