Nam quod attinet ad soli electionem in statuenda metatione, primum locum habent quae ex campo in eminentiam leniter attolluntur, in qua positione porta decimana eminentissimo loco constituitur, ut regiones castris subiaceant. porta praetoria semper hostem spectare debet. secundum locum habent quae in plano constituuntur, tertium quae in colle, quartum quae in monte, quintum quae in loco necessario, unde et necessaria castra dicuntur. praecipue observari debebit via quae lateribus castrorum supersit. ceterum quocumque latere flumen sive fontem habere debebunt in qualicumque positione castrorum. iniqua loca, quae a prioribus novercae appellantur, omni modo vitari debent; ne mons castris immineat, per quem supervenire hostes aut prospicere possint quid in castris agatur; ne silva celatura hostes adiaceat neve fossa vel valles per quas obrepi castris occulte possit; ne vicini fluminis torrentis subita tempestate castra inundata intereant. meminisse oportet in hostico ascensus valli duplices et frequentes facere et tormentis tribunalia exstruere circum portas, in coxis in loco turrium. maxime instruendum erit vallum tormentis ab eo latere quo novercae, si vitari non potuerunt.
(Pseudo-Hyginus, De munitionibus castrorum 56-58)
Concerning the choice of terrain for the establishment of the camp; first they choose a site which rises gently above the plain, on a distinctive rise and the porta decumana is set at the highest point so that the area is dominated by the camp. The porta praetoria should always look towards the enemy. The second place is situated on a flat plain, the third is on a hill, the fourth on a mountain, the fifth in whatever place is necessary, from which it is called an “unavoidable camp”. It should be particularly noted that a road should be built which is longer than the sides of the camp. Whatever the position of the camp there should be a river or spring on one side or the other. Unfavourable positions, which were called mothers-in-law by our ancestors, should be avoided at all times: the camp should not be overlooked by a mountain from which the enemy could attack or see what is going on in the camp; there should be no forest nearby to conceal a hidden enemy, nor gulleys or valleys through which the enemy may secretly approach the camp; nor should the camp be near a fast-flowing river which might flood and overwhelm the camp in a sudden storm. In hostile territory one must remember to construct numerous double width access ramps up to the rampart and to build artillery platforms around the gates, on the projections at the corners and in places on towers. In particular the rampart should be fitted out with artillery on any side which is a mother-in-law if this cannot be avoided. (tr. Catherine M. Gilliver)
Crambe Repetita 