Ok, a bunch of ideas already, cool!
Some comments and clarifications.
I'm the DM. I'm giving you a specific setup of something the orcs are currently working on in the current campaign area. Although taking the keep would be nice, coping with the forest-fighting is more of a priority right now.
The PCs are human renegades fighting the orcs along the river. The term 'Vietnam' has popped up more than once in my group. I wonder why...
Re burnin' and choppin':
We're working on it. The damn forest is still a couple million square miles big, so we will be a while.
Joshua Dyal:
Please let's ignore magic support for now, as I said. Legates and channelers are rare in the orcish army (much rarer than spellcasters on the elf side), plus too setting-specific for most posters here to make sense out of them.
Rodrigo: Diverting the river, nice. Feints work to some extent, unfortunately splitting troops up into smaller units invites attacks from elven guerrilla and human renegade bands.
derelictjay: Ramps might work in conjunction with diverting the river to actually get at the keep, which sits on a rock among cataracts.
Nifft: Psychic screening by scattering fires around the place and torturing prisoners? Very nice. Very appropriate. I wish we could get our hands on enough live elf to make that an even more effective and widespread tactic ...
JimAde: There's a fortified camp already. Building a stronghold upriver from the keep is brilliant but will require more troops, we're hoping for reinforcements. Meanwhile, how to hold out and keep us from being cut off?
Thornir: What we need to keep the river clear of is blockades. Chains spanning the river, submerged wrecks of ships the enemy takes from us that blocks passage, that sort of thing. Obviously we're not holding every single mile of those accursed riverbanks, if we did that problem with the keep would long have been solved. Landing sites we've got, fenced, too. Since we've started manning them with a couple of hunts (60-70 orcs) each, they tend to stay. Attrition is going on all right. Not sure how long we're going to have to hold out - maybe we're here only as a thorn in the elves' side by now, maybe another couple of legions are getting ready to ship upriver as we speak. Who knows...
derelictjay: We've tried paths, in fact we keep making paths. Dunno what it is, paths tend to curl around on themselves, wander off somewhere deadly (for us) or disappear entirely when we're not looking. It's not like everyone who goes out of sight of the river is a dead orc, but we've found it's safer and less disorienting to stay near the waterways.