[RPG] Middle age transportation by river questions

In D&D 5e, the daily rate of mounted overland travel is generally the same as on foot, because horses get tired and adventurers carry a lot of heavy equipment.

See the section Special Travel Pace in the DMG (p. 242–243). This section starts:

The rules on travel pace in the Player’s Handbook assume that a group of travelers adopts a pace that, over time, is unaffected by the individual members’ walking speeds. The difference between walking speeds can be significant during combat, but during an overland journey, the difference vanishes as travelers pause to catch their breath, the faster ones wait for the slower ones, and one traveler’s quickness is matched by another traveler’s endurance.

In the same section, the rule is:

• In 1 hour, you can move a number of miles equal to your speed divided by 10.

and then:

• For a fast pace, increase the rate of travel by one-third.
• For a slow pace, multiply the rate by two-thirds.

So an unencumbered horse with a speed of 60 could theoretically travel 6 miles in an hour at a normal pace. At a fast pace (a gallop), 8 miles per hour. That's "twice the usual distance for a fast pace", where "usual" means a creature with a speed of 30. This suggests that a riding horse with no rider, traveling alone, can cover 48 miles per day at a normal pace.

So the rule that "a mounted character can ride at a gallop for about an hour, covering twice the usual distance for a fast pace" seems to exist to allow for mounted travelers covering short distances quickly by using the mount's speed instead of "the usual pace", for up to an hour each day.

So, according to the rules, a traveler on a horse at a normal pace (3 miles per hour) will cover about 24 miles in an 8-hour day. If you make the horse gallop for an hour each day (fast pace for a horse being 8 miles per hour), that range increases to 29 miles. That's within the realm of what you would expect in real life, with a fast horse on good roads in fair weather.

Variant: Encumbrance

If you're using the encumbrance rule, a Riding Horse needs to be carrying less than 80 lbs of rider and equipment to get its full speed of 60. Loaded with between 80 and 160 lbs it has a speed of 50, and carrying between 160 and 480 lbs (its maximum carrying capacity) it has a speed of 30. A 200 lb adventurer in chainmail with a dungeoneering pack, longsword, and shield weighs in at about 325 lbs, so under this rule a horse's travel pace is usually the same as an unencumbered adventurer on foot.

As you said you're all right with consulting the lore of previous editions for an answer, I'd suggest taking a look at Planes of Law from Planescape, if you can get hold of it. (Not just for this particular question, but because it has an excellent presentation of Baator as a whole.) Fires of Dis would also be a great reference for Avernus and Dis in particular. The 3e Manual of the Planes also has a good overview of Baator that could help flesh out your planned adventure.

Anyway, on your particular question. According to a map provided in Planes of Law, as well as the individual descriptions in separate layers, there are the following inter-layer paths between layers (any without descriptions are referenced only on the map):

• Avernus -> Dis: On foot by passing through a massive gate within Tiamat's realm, or via a portal from the city of Darkspine to the City of Dis.

  • Page 14, PoL: Baator

    The Styx runs right through the middle of Avernus. It used to be right on the edge of the layer, but, due to the success of the recruiting plan for the gate-towns like Ribcage and Darkspine, Baator's been gaining territory.

  • Pages 16-17, ibid.

    Tiamat, the Lady of Dragonkind, guards the only known stable entrance to the next layer. It's only through her lair that one can arrive in the verdigrised plains near the Iron City of Dis. The mere thought of approaching her lair's enough to turn back most berks; those that don't go back are definitely the ones with something to prove. It's for this reason that Tiamat (or her guards) even bother to speak with those who approach - they know that the cutter doing it's either extremely powerful, extremely brave, or extremely driven. The guards to the Second Layer appreciate all three of these qualities, and may very well bargain with any planewalker who summons the courage to approach them.

    Of course, if a berk comes in too cocky, the dragon guards might just decide to fry him outright. No one appreciates a sod who doesn't know when to keep his bone-box shut.

  • There's a broad description of Tiamat's lair in Fires of Dis pp.32-33, too much to reproduce here.
• Dis -> Minauros: Portals from the City of Dis to either Jangling Hiter or the City of Minauros.
• Minauros -> Phlegethos: Portal from the city of Minauros to the city of Abriymoch.
• Phlegethos -> Stygia: Portals from Abriymoch to the city of Tantlin.
• Stygia -> Malbolge: Portals from Tantlin and the divine realm of Ankhwugaht (Set's realm) to unnamed locations in the latter layer (as Malbolge has no settlements).
• Malbolge -> Maladomini: Portals from an unnamed location to the city of Malagard.
• Maladomini -> Cania: Portals from Malagard to the city of Mephistar.
• Cania -> Nessus: The portal you mention referenced in Wikipedia, which was also referenced in this work.
  • Page 25, ibid.

    The only known portal from Cania to Nessus (the ninth layer) leads directly to the fortress Malsheem. To enter the portal, a body first has to get to the bottom of a deep glacial pit in Cania. Then, he has to beat off the 9,999 gelugons standing guard at the portal. Assuming a berk somehow gets past this little obstacle, he must plunge into the icy lake at the bottom of the pit and swim downward through the near slush. Swimming is no problem for fiends, who don't really need to breathe anyway, but the 1,001 fathoms a mortal has to swim through might prove rather...difficult. At the bottom of the lake is a portal of ice and corrupted silver, and through the portal is Malsheem.

Further, there are direct portals from the Outlands to the city of Grenpoli in Maladomini, as well as from Sigil to both Grenpoli and Malagard. And of course, the main portal from Ribcage in the Outlands to Darkspine in Avernus. From Planes of Law: Baator, page 5, a general description of the state of portals out of Baator:

Most of the known gates [out of Baator] are two-way and are extraordinarily well guarded, preventing both entrance and egress. Anyone hoping to escape via these gates had better think twice, because the baatezu won't when they catch the poor sod. Most of these portals are build inside the fortresses of the major fiends of the layers. No one knows if the fortresses sprang up because of the portals, or if the portals just happened to appear in the fortresses after construction. 'Course, no one's really asking.

And from page 4, a description of the gate between Ribcage and Darkspine; this is from the perspective of entering Baator, but there's still some relevant portions:

The best route in is via a gate located in Ribcage, that "quaint little village" on the edge of the Outlands. The Cursed Gate, as it's called, is in a heavily fortified part of town; in fact, it's accessible only through the citadel of Lord Paracs, the ruler of Ribcage.

...

...the lord of Ribcage and the baatezu guards on this side of the gate have come to a tidy little agreement, one that profits both sides and doesn't bend the laws too far. Basically, the two parties bob travelers through the gate for whatever they can get...

Also, just as a note that's not especially relevant for you, earlier works suggested that the River Styx only connected to Avernus (as it connects to the first layer of all the Lower Planes) and Stygia. Of course, this isn't binding on later lore by any means, and that passage in the 5e DMG definitely does suggest otherwise as of 5e.