When a defeated landmark grants you a free spire, which spires can you choose from?

You can only construct spires which you have access to. If you are playing the Heirs, Grovetenders, or Brawnen and have not purchased a specific fortress advancement, you will only be able to construct a basic spire. Spires that cannot be constructed during the onslaught phase, such as the Brawnen Drilling Outpost, cannot be selected for this reward.