Complex sentence: As noted above Agricola had advanced far enough to threaten their vital interests, indeed his strategy was no doubt formulated with the end in mind of forcing just such an engagement as Mons Graupius.
Split into simpler sentences: As noted above, Agricola had advanced far enough to threaten their vital interests. Indeed, his strategy was no doubt formulated with the end in mind of forcing just such an engagement as Mons Graupius.