Usually a state temple was located in the centre - or at least within the compound - of a royal city, such as Bakong in Roluos and Bakheng in Yashodharapura, which was the first city in Angkor. Similarly, Prasat Prang (at Prasat Thom) was the state temple in Koh Ker, and Pre Rup was inside a new eastern city area of Angkor under King Rajendravarman II. Halfway between this eastern area and the central former Yashodharapura, which is partly overlapped by the later Angkor Thom, there is another state temple of the tenth century, Ta Keo. This led to the assumption that there was another city founded between the later Angkor Thom and the East Baray. But there is little evidence for such a city "Jayendranagari" serving as the residence of Ta Keo founder Jayavarman V (968-1001), except one tower south-east to Ta Keo. This small Prasat, with an uncommon single door opening to the south, is similar in style to the corner towers of Ta Keo. It is sometimes called "Prasat Top". Be aware, there are three more structures of the same name in Angkor. Maybe this small building, located halfway between the later compounds of Ta Prohm and Ta Nei, marked the south-east corner of that hypothisezed city Jayendranagari. As already mentioned, apart from this small structure, there is little evidence for the existence of that city.
A short break between between visits of Ta Prohm and Ta Keo is enough to see this small tower, at any time of the day. You are in the core area of Angkor here, so do not come without an Angkor ticket, though nobody will check it at this Prasat Top.