How do you station a hero into a town/outpost?

[TwoTonTuna]

I notice the AI defending from behind town walls. But when my heroes guard the 'gates' of my town, they always meet the invaders on an open field.

How do I station my heroes to fight from behind the walls of a town/outpost?

[Pim]

you just have to be very close to the walls and it should auto put you in a field with the walls, position yourself in a way the AI has to attack you and the wall at the same time

[Rogue Arcadia]

So, Pim is mostly correct and if you're already doing it, great. However, if you still don't know how to do it, and for others looking for help with this as well, you will notice your team flag at the bottom of the city. You click on the flag and if the unit is on top of it, they will, effectively, "absorb" all created units, like anything that normally goes to your garrison will go to your hero, and you can even check this by hiring a second hero, moving them a square or two away and then trying to go "into" town with them, the normal "click the town" way, and it should swap over to the other hero. Effectively, the garrisoned hero becomes your "vending machine" and your shield as well.

Also, there are a handful of exceptions to the "hero default gets the unit" and it's units that show up in the garrison on their own. Key examples are the Bears, Skeletons and Dust Devils, if you try and use anything that just "shows up" each turn as a "test" for if something is garrisoned, it won't work, it should almost always pop into the garrison as you're not "making it for your hero" in specific, you're more or less making it for your army. Now, if you want that on your hero, drop and drag every time they show up. Thought I'd mention it as an exception.

Edit: Click on the flag of the city while using a hero. I forgot to mention that the hero you want to be garrisoned needs to be selected before clicking on the flag. For as long as they stand in front of that flag, they become like a vending machine, and if you notice, that's where the CPU stands to defend towns.