Calenhand, Min-rimmon, Erelas, Nardol, Eilenach, and Hmon Din

[Éomond]

Where these just towers/cities of Gondor? What are they? Are they every metion much or describe by Tolkien?

[Ithrynluin]

These are the beacon-hills of Gondor. Great fires were lit on top of them when there was an emergency (a call for arms, other need of haste).

[Mablung]

They were strung up between Minas Tirith and Eastfold (?) so any attacks on either Rohan or Gondor could be seen by the other.