Housecarls formed the professional core of the English army’s shield wall at Hastings. They were full-time, heavily equipped soldiers who served powerful English kings and nobles, unlike the more lightly armed temporary levies of the fyrd.
At Hastings, the English army fought mainly on foot. Housecarls typically carried shields, helmets, mail armour and the formidable Danish-style two-handed battleaxe. Front-rank soldiers locked their shields together to create a dense defensive formation, while other troops stood behind them with axes, spears, javelins and bows.
The shield wall was not made up exclusively of housecarls: fyrd levies also participated, and the exact composition varied across the line. That distinction explains a common mix-up in this question. Fyrd describes a broader military levy, while housecarls were the trained professional soldiers who provided the army’s elite backbone. Archers and knights were not the core of Harold’s infantry formation; the English army had few mounted troops and relied heavily on foot soldiers.