BoLS on 40K faction-specific fortifications. It's a list of unofficial ideas for the Tyranids in the form of a D33 table, and might help non-40K players too.
- Grown: 1) hyphal outcrop; 2) chitinous reef; 3) erupted lithocyst; 4) bioship anchor spine; 5) coiled atmospheric lash tentacle; 6) zoanthropic warp foam
- Evolved: 1) fused rippers; 2) swollen carnifex; 3) woven barbed strangler cluster; 4) rooted malefactor; 5) anchored sterile mioetic spore; 6) heavy mycetic spore
- Induced: 1) cult construction; 2) mind-slaved megafauna; 3) petrified local flora; 4) cooled magma updraught; 5) crystallised cumulus; 6) meteoritic ejecta
- Produced: 1) abandoned brood nest; 2) shed carapace; 3) superheavy dung heap; 4) clotted ichor spout; 5) viragon silicate cast; 6) regurgitated biomass
- Injured: 1) part-emerged mega-mawloc; 2) curled up biotitan; 3) fallen harridan; 4) severed proboscis; 5) charred bioship wreckage; 6) calcified warp shadow
- Overrun: local equivalent, either 1) infested, 2) part-consumed or 3) developed
For suggestions on modelling bio-terrain try this post at Confessions of a 40k addict.
For modelling the overrun locations, or more typical defences, there's an interesting idea at In the Grim Cheapness of the Future..., where Virtual Stranger suggests beach sets.