The new location of the harbour and trading site was accompanied by the establishment of a Toompea fort at the end of the 11th century.

Its exact location is not known as the surface area of the two-level limestone plateau at Toompea extends over 70 000m². Presumably it was situated on the higher level on the northern side of the limestone plateau (ca 55 000 m²) which made it a fort with the largest surface area in Estonia. As the best farmlands were situated south-east of the present city near Iru, the castle at Toompea must have belonged to the given inhabitants.