Castles in Middle Earth

[Iargwath]

Aren't "Strongholds" more commonly referred to as Citadels?

There is a comprehensive list of Fortresses/Strongholds over at The Encyclopedia of Arda. Here is their list, and some small detail behind each stronghold. Each "Castle" is in *BOLD*.
[*]Angband (First Age):- Citadel constructed by Melkor[*]Angrenost:- Elvish name for Isengard[*]Barad Eithel:- Fortress of Fingolfin and Fingon[*]Barad-dûr (Second Age):- Sauron's Dark Tower[*]Belegost (First Age):- Elvish name for a Dwarven citadel in the Blue Mountains[*]Dol Guldur (Third Age):- Stronghold of Sauron in Mirkwood
[*]*Durthang:- An old Castle in the North of Mordor*[*]Formenos:- Nothern stronghold of Fëanor in Valinor
[*]*Hornburg:- Castle guarding Helms Deep*[*]Isengard (Third Age):- Sauruman's fortress[*]Lugbúrz:- A name for Barad-dûr in the
Black Speech[*]Minas Anor (Third Age):- The original name of Minas Tirith[*]Minas Ithil (Third Age):- Isildur's Tower of the Moon, later called Minas Morgul after it was captured by the Nazgûl[*]Minas Morgul (Third Age):- Refer to Minas Ithil above[*]Minas Tirith, Beleriand (First Age):- The Tower that guarded the Pass of Sirion[*]Minas Tirith, Gondor Third & Fourth Age):- Gondor's Tower of Guard after the destruction of Osgiliath[*]Moontower:- A name for Minas Ithil[*]Mount Fang:- A name for Orthanc[*]Mundburg:- A name given to Minas Tirith by the people of Rohan[*]Nargothrond (First Age):- Hidden Fortress on the River Narog[*]Orthanc (Third & Fourth Age):- The Tower within the Ring of Isengard[*]Tower of Amon Sûl:- The Ancient Stronghold of Arnor, built by Elendil[*]Tower of Cirith Ungol (Third Age):- Originally a Gondorian watchtower, guarding the Ephel Dúath. Was later captured by Sauron's forces[*]Tower of Guard:- A name for Minas Tirith[*]Tower of Orthanc:- Originally built by Gondorians, and was given to Sauruman by Beren[*]Tower of Sorcery:- Title for Minas Morgul[*]Tower of the Moon:- Antoher name for The Tower of Minas Ithil[*]Tower of the Rising Moon:- Another name for The Tower of Minas Ithil (used in reference to it's geography)[*]Tower of the Setting Sun:- Another name for Minas Arnor[*]Towers of the Teeth:- The gateway at the Pass of Cirith Gorgor[*]Udûn:- Sindarian name for Utumno (fortress of Melkor)[*]Utumno (before the First Age):- Stronghold of Melkor in the Northern World

This is just a brief look at the Stongholds/Fortress' of Middle-earth. There are more detailed entries about each of these buildings at THIS page. Look directly under the "PLACES" section on your left-hand side.