A.) illusions can affect all senses EXCEPT tactile. While advanced illusions can produce thermal sensory effects (hot/cold) and they can produce pain, no illusion can provide a false sense of solidity. If you stick your hand through an illusion, you will feel nothing, and will know it's an illusion.
B.) you get a saving throw when an illusion lacks a sensory input that it should have. To use your illusory wall example? I would actually give NO SAVE against it! Walls don't normally make noise, have a distinct smell, or produce any other sensory input that would be missing. But if a PC went around knocking on all the walls, they would detect the illusion automatically. An illusory orc with just visual (phantasmal force) might grant a save if a PC is close enough to hear its movements (footfalls, armor creaking, etc), or if they had keen smell and should smell it.
C.) illusions in combat cannot deal damage, or indeed have any "real" effect on creatures. An illusion can be made to appear to fight, but all its attacks will miss (as a hit would have no tactile, and reveal the illusion). Attacks against the illusion target an AC based on how good the illusion is, and a hit will reveal it to be an illusion.
D.) illusions always have a visual component...and that remains even if you know it's an illusion! I have heard of a trick of "create an illusory wall, but it only blocks line of aight for enemies, it's transparent to me since I know it's illusory"...doesn't work in my campaign.
You can run this setting with no changes to vanilla 5e dnd. This will work fine. However for an optimal experience I really suggest a few changes to stock rules
I wrote a series of blog posts in this (part1, part2, part3) that outlines the reasons behind the changes I recommend and how to implement them.
The tl;dr is a age of sail campaign works best if you run a relatively low level campaign, limit spells that directly effect seafaring (the The Lady’s Law is an example of this) and adopt an alternate rule system for naval mass combat.
I especially recommend using alternate rules (Blood and Plunder ) for ship to ship combat dor the following reasons.
- The ship models are gorgeous
- The level of historical accuracy is very high
- The game mechanics are smooth and easy
- The length of a ship engagement (around 45 minutes) fit well into a play session.
- The basic unit of abstraction (one model = one actual person, a dice rolled is an actual person making an attack) had the potential to mesh well with D&D as opposed to other systems that generally had a higher level of abstraction.
- Naval combat was designed with 1-3 ships on each side, but a higher level of abstraction game (Oak and Iron) was available that simulated fleet on fleet actions. Haven't used that yet but good to know it is there if needed
- The game was specifically built around pirates, and thus the thematic elements supported were very synergistic
- It was a living game, with constant new content and an engaged and growing community
There are other reasons as well. It's generally a great game.
It took only a little work to houserule a way to "import" the player characters into the B&P combat system, and we were off and running.
in addition, since sea voyages play such an important role in nautical adventures I created a set of Sea Voyage Rules that provide a framework for running such a journey.