I don't think it breaks anything to make this one land untap a lot more than normal. I think it'll sometimes be useful, but not often enough to make it need to be uncommon.

Wording is tricky: at the moment as a triggered ability, this accelerates you by one mana because you can tap the land within that main phase in response to the trigger. If it were phrased more like Seedborn Muse then it wouldn't work that way, but there isn't much precedent for "automatic" untapping of things in phases/steps other than the untap step.

Right, "untap fortified land" is a great idea and a natural fit, and you found an expressive flavour. I provisionally think we have a common fortification :)

It could even be "at the beginning of every second main phase" or "at the beginning of your second main phase" and still be interesting.

Heh. I like this. We probably need this to say "Each other player's main phase". As it stands right now, it will give you an extra untap during your turn... and that's arguably too cheap for .

I also like how this works with other fortifications. We may want to change the flavor later to reflect this, because I assume that this one is being dropped right into the 'fortification deck'.

Hmm, it is a bit better than Ley Druid, I guess.

Looking at the fortifications in common that give you mana, and am planning on adding 2 to the skeleton. I like this card, but It's not going to do much of anything in most draft archetypes. I'm planning to bump it to uncommon, where it's better suited.