The photograph was taken on a very wet day, but the essential features of the fort can still be seen. The remains of the fort lie between the cycle route / footpath (the Rob Roy Way, from which the photograph was taken), and the river. The footpath follows the line of a former railway embankment, the construction of which obliterated part of the fort's southern side. Likewise, the river, changing its course over the centuries, has eroded away the fort's north-eastern corner.
Click on the end-note title for several other pictures of the fort.
The present photograph shows the western side (in other words, the fort's interior is on the right). Near the centre of this picture, the location of one of the fort's entrances is clearly visible: at that point, the earthwork bends in towards the interior. This entrance is located at the middle of the fort's western side; there was another entrance at the middle of the eastern side.
(The eastern side is less clearly visible, but is shown in
NN6107 : Remains of Bochastle Roman Fort and
NN6107 : Bochastle Roman Fort: south-eastern corner.)
This is one of the so-called Highland Line forts; of these, David Breeze writes (where I have added grid references) that they "are sometimes called the glen-blocking forts, an apt description of their location for many were placed in the mouth of the principal glens. The chain of forts stretched from Drumquhassle (
NS48468748), near Drymen, perched on the high ground overlooking the southern reaches of Loch Lomond, north-eastwards to the fortlet at Inverquharity (
NO406580). The location of the forts at Bochastle by Callander, Dalginross (
NN773212) near Comrie and Fendoch (
NN91962830) in the Sma' Glen is particularly interesting, for each is placed in the very mouth of the glen before it narrows and fully enters the Highlands" [from David Breeze's book "Roman Scotland: Frontier Country" (Historic Scotland; 2006)].
See
NS4887 : Site of Drumquhassle Roman Fort for the most southerly of the forts just mentioned.