They may be constructed with covered ways, and sometimes it may be found advantageous to secure them by retrenchments.
A _detached work_ may be made in any form deemed best suited to the site.

Being but remotely connected with the fortress, the latter will exercise but slight influence on the character of its plan or construction.

They are usually of limited extent and slight relief, partaking much of the nature of field-works.[45] [Footnote 45: The general principles of permanent fortification may be best learned from the writings of Cormontaigne, St.Paul de Noizet, and Laurillard-Fallot.

A list of valuable books of reference on the several branches of military engineering will be given at the close of the next chapter.].