Ymseborg, Medieval castle in Töreboda Municipality, Sweden.

Ymseborg is a ruined hilltop fortress on the western shore of Lake Ymsen in Bäck parish, Töreboda Municipality. Its core was a square grey-stone tower with 10-metre sides, and the whole plateau, about 20 by 30 metres, was ringed by a rampart or curtain wall. A lower plateau to the south, cut off by a moat, probably formed an outer ward, and a dammed stream created a marsh on the landward side.

Ymseborg is probably from the early 13th century, though its age is unknown and it has never been thoroughly investigated. It was owned by Filip Knutsson in the mid-1200s and first appears in records in spring 1281, when Magnus Ladulås wrote a letter from the castle spelling it Wmpsæborg. It played a role in the Third Folkung uprising (1278–1280), and Gerhard I of Holstein was brought here after his capture in Skara. The castle was likely destroyed in 1282.

The name appears as Wmpsæborg in a letter Magnus Ladulås wrote from the castle in spring 1281. The Danish Wikipedia links the site to Katarina Sunesdotter, and Jan Guillou placed Ymseborg in his novels Riket vid vägens slut and Arvet efter Arn. Gerhard I of Holstein was held here after his capture in Skara.

The site is accessible via marked paths and is best visited during daylight hours for good visibility of the structures. Sturdy footwear is recommended for exploring the grounds comfortably.

Ymseborg was quarried in the 18th century for stone to build the nearby Ymsjöholm manor, so much of the fortress has been carried away. The site also appears in Jan Guillou's novels Riket vid vägens slut and Arvet efter Arn.