D&D 5E Running several combats in a row

[Quickleaf]

Yes. They'll fly onto an enemy tower and fight their way down. They don't know exactly what it will entail (actually, at this point, neither do I!) but they'll be clear that they will be fighting several waves in succession as they try to descend the tower and the enemies try to stop them.

The transition points between each wave are an opportunity to control pacing, reduce grind by introducing twists/interesting narrative developments, and also a way for you to adjust difficulty dial on the fly (e.g. giving party a few rounds to prepare for next wave is not a rest, but long enough to reposition and cast spells).

 

[Zaukrie]

The transition points between each wave are an opportunity to control pacing, reduce grind by introducing twists/interesting narrative developments, and also a way for you to adjust difficulty dial on the fly (e.g. giving party a few rounds to prepare for next wave is not a rest, but long enough to reposition and cast spells).

Great advice here.

 

[Koloth]

Yes. They'll fly onto an enemy tower and fight their way down. They don't know exactly what it will entail (actually, at this point, neither do I!) but they'll be clear that they will be fighting several waves in succession as they try to descend the tower and the enemies try to stop them.

Then it can be assumed that they arrive prepared for a long slog and have brought supplies they think necessary. (Be prepared to fudge a bit on this. If the Players forgot to bring something(s) the characters would have thought to bring, let it show up IF the characters have enough spare cash. If the party is broke, they couldn't have bought it to bring it.)
If they get surprise(or it is assumed), then the party can control pacing to some extent. A surprised tower guard will take a few minutes to become aware, react to any alarms and respond. But as I mentioned, if the party delays too long after defeating wave #X, then wave X+1 can control how the next encounter starts, and may even achieve surprise on the party. Depending on tower layout and party abilities and injuries, possible a scout/stealth character could advance a bit and serve as an alarm. And maybe even provide a couple rounds of delay. A fireball in the face might slow up a responding group of guards for a few rounds while they assess what they are dealing with. But any delay will be fairly short as the guards are on home turf and will probably feel they have the advantage.

If the encounter literately blows up in the party's face and results in a TPK, you can always do a GM hand wave reset.

 

[Li Shenron]

I want the battles to be very tough but winnable.

What do you mean by "very tough"? Personally I think that the general quality to make a situation feel tough is uncertainty. Something feels tough when you don't know you're gonna succeed until you do, and in the meantime you don't know if you're running out of time, resources, fatigue, or if there's a pending chance of sudden failure (other random or due to a possible mistake on your part).

Now, for some players a "very tough" combat in a RPG can mean pretty much grinding each other resources not knowing who's going to run out of them first. You can't have multiple encounters like this unless you provide a "reset" (total or partial) of resources for the winners between combats.

So if I wanted to achieve the feel of "very tough" all the time here, I would look into different opportunities:

1) Make all the encounters feel like one: let the players know that the other waves are coming quickly so that they start feeling pressed and concerned immediately about conserving resources. In theory, you could use the encounters building guidelines to balance everything as a single battle, keeping in mind that the fact that parts of the enemies enter combat with a delay decreases the difficulty, so you have to estimate how much to compensate for that to achieve the wanted difficulty. This should be the easiest option, but at the cost of having less room for surprises, and that you won't have a "barely made it" feeling repeated multiple times.

2) More challenging for you, another option is to design encounters not based on resource grinding but on key gamechangers. For example, for every wave there may be 1-2 ideas to discover that allow a quick win: collapsing a structure where the enemies are standing, opening a floodgate which swipes them away, setting the enemies on fire (classic scenario: torching a giant web full of spiders)... If you make the encounter feel slow and sluggish enough, giving the feeling that on a strict resource grinding tactic the party will lose, it forces the players to think outside the box while giving them time to find decent ideas.

 

[Horwath]

THIS is one of those scenarios where "short rest" classes are doomed, unless you can get 1min Short rests, those running combats are awful for those classes, long rest classes have resources to stretch somewhat between several combats, but in general, you should get Short rest after every (meaningful) combat.