We may think of the apologist as constantly walking up and down on or near the outer defenses of the fortress. This will give the other occupants time to build and also enjoy the building. The others too must defend, but not so constantly and unremittingly. The apologist too must rest and must enjoy the peace of the fort, but his main work is to defend and vindicate.
Cornelius Van Til, Christian Apologetics, 2nd. ed. Edited by William Edgar (P&R, 2003), 22-3.