I had heard okay things about this AI mod:
http://forums.civfanatics.com/showthread.php?t=309696 , it turned out fairly easy to apply to the game, and I noticed a substantial improvement in the level of sea invasion jeopardy, whether in normal games or in FFH2 official scenarios.
An example if playing the Black Tower with and without this mod going, is that most people should manage to hold a particular island as their main base of operations... before the mod was added, sure I'd see the occasional naval unit sniffing around, with the occasional blockade to ruin my day, but after the mod, I'd find ship based stacks of doom showing up now and again and me having to scramble defenses to concentrate on whichever coastal region was in the most danger.
I am sure there are other good AI packages out there, but this one seemed to up the challenge for me in terms of not neglecting navies.
Still, on the main point of this thread, if you can find ANY method of "turtle" type defensive play, sure you can try to do an Altar victory. Small amount of turf is needed, unlike the Tower victories where you need a whole lot of mana (and therefore the territory to do that). Again, that turf is often an island, but sometimes you'll find on an Erebus map that you're able to hold onto a mountain-protected kingdom pretty well by keeping focus on defending a couple of bottlenecks, though you might be harassed now and again by rare flying units, like some of the upper tier Mercurian ones (Ophanin chariots? Heralds too? I forget).