I had an NPC spellcaster use forbiddance to protect his study chambers.
The spell was cast twice, once in the shape of a letter "S" and again in the shape of an inverted "S", interlocked with the first one. The positioning of the two spells changed each time the NPC cast them. A stranger would have to cross the spell multiple times unless they somehow knew which squares to use to move across the area. Proper placement of illusionary spiked pits gives an intruder something else to think about, leaving them less likely to concentrate on the forbiddance spells.
Although reading the RSRD just now, the area is "60-ft cube/level (S)" and I seem to recall playing it as "6 10-ft cubes/level (S)" instead. Maybe it changed from 3.0E to 3.5E? Double-hmm...