The Red Castle once stood directly on the water, its walls meeting the Mediterranean surf. That changed in the 1970s, when the construction of Al-Shat Road, a seafront highway, severed the castle from the sea and created the artificial Saraya Lake in the gap. What had been a coastal fortress became a landlocked monument bordered by a man-made pond. The building forms an imperfect square, its four sides measuring between 90 and 140 meters, flanked by Martyrs' Square to the southeast, the Central Bank of Libya to the northwest, and the old Souq al-Mushir neighborhood around the Karamanli Mosque to the southwest. The castle's mass dominates its surroundings, yet the lake that separates it from the shoreline gives it an oddly stranded quality, a fortress designed for the sea now gazing at it from behind a highway.

The parade of flags over the Red Castle reads like a syllabus on Mediterranean empire. After the early Arab conquest, it passed through the Abbasids, the Aghlabids, and the Fatimids, who upgraded it into a proper royal residence in the tenth century. The Zirid emirate held it, then the Normans from Sicily from 1146 to 1158, then the Almohads, then the Hafsids in overlapping and contested stretches through the fifteenth century. In 1510, the Kingdom of Aragon captured Tripoli, and much of the castle's current structure dates from the Aragonese period and the subsequent rule of the Knights Hospitaller, who held it until the Ottoman Empire expelled them in the Siege of 1551. Ottoman rule continued through the Karamanli dynasty until 1835, adding yet another architectural stratum to a building that was already ancient.

When Italy colonized Libya in the early twentieth century, the castle underwent its most dramatic aesthetic transformation. Italian architect Armando Brasini created a new entrance portal designed to look ancient and built the iconic arches above the eastern Saint James bastion, originally facing the open sea. Brasini had envisioned an even grander scheme with a second level of arches above the first, but only one tier was completed. In the 1930s, Governor Italo Balbo made the castle his personal office, embedding Italian colonial administration within walls that had served North African, Arab, and Ottoman rulers for centuries. A sculpture of Saint George from the 1920s renovation still adorns the Saint George bastion, a Christian knight watching over a city that had long since become predominantly Muslim.

The Red Castle has housed a museum since 1919, making it one of North Africa's longest-running cultural institutions. British authorities, who administered Libya from 1942, protected the building and expanded the museum in 1948. Under Muammar Gaddafi, the museum was refurbished between 1982 and 1988, and the square outside was renamed in his honor. During the First Libyan Civil War in 2011, the castle and museum suffered only minor damage, a small mercy in a conflict that ravaged much of the country. The museum closed in 2011 amid ongoing instability and did not reopen until December 2025, its collections spanning Roman antiquities, Islamic art, and artifacts from the castle's own kaleidoscopic history. Through every political upheaval, someone has thought to preserve what the walls contain.

Walking through the Red Castle's interior streets, where the light falls differently depending on which century's architecture you are passing through, the building feels less like a single monument than a neighborhood compressed into one structure. Aragonese bastions sit atop Fatimid foundations. Italian arches frame Ottoman courtyards. The very name, the Red Castle, comes from the warm hue of its stonework, which takes on a deep crimson tone in the afternoon light. Tripoli has been destroyed and rebuilt more times than most cities bother to count, but the castle has simply absorbed each transformation, growing larger and stranger with every century. It remains what it has been since at least the fifth century: the reason anyone bothered to fight over this particular stretch of coast.