Alright, it's time to move on!

Though we are in the overworld, we can really only go to one other place at the moment - this cave. Let's just do it.
Cave of SpiritsWalking just a few steps into the cave leads to our first encounter.. and it's a scripted one too!
Battle 1So a quick rundown of combat actions: we can pick between Fight, Tactics and Run. Fight brings up a submenu of options, Tactics allows you to choose between a bunch of auto combat modes or manual battle, and Run does what you think it would. Currently we don't have Tactics because we only control Artur, and always have to issue commands to him.
As for Fight's submenu, it provides options to Attack, use Magic, Defend, or use Item(s). If you've played any RPG, odds are you know what all of these commands do. Honestly, even if you haven't, they're fairly self explanatory.
Given that this is the early game, Artur doesn't really have any options besides Attacking or Using items to heal.
This particular fight goes fairly trivially, and gives equally trivial rewards.

Artur: Huh?!(Rejoice, the presumably silent character spoke!)
Annie: I came anyway!
Steiner: Oh... It's Annie.
Annie: "Oh... It's Annie." What kind of greeting is that?!
Steiner: Annie, why are you here? You're not thinking about going with us, are you?(Wow, Steiner really doesn't like Annie being here)
Annie: Why, Steiner?!
Annie: Do you think I would be a nuisance if I came along?!
Steiner makes a head shaking motion with his body, as if to say no
Annie: Do you feel the same way, Artur?
Artur also does a head shaking motion, except with his, well, head
Annie: Really??? Are you sure?? I won't be in the way?
Both Steiner and Artur proceed to perform the head shaking motions they did before
Annie: But maybe I shouldn't... I'm just a weak girl... I wouldn't want to hinder your journey.
Annie: If I get in your way, then you won't be able to do the favor for my father.
Artur nods his head(This seems like a dumb thing to do, and it contradicts all the fervent head shaking so far. So does Artur want Annie on the journey or not?)
Annie: Hah! I knew that's how you felt! Just like every other MAN!
Annie quickly moves upto Artur
Annie: You think you're so strong that you don't need a woman's hand, huh?!(Oh I see, she's going to act like a spoiled brat in response to Artur's poor gesturing. That's cool.)
Annie: Hmph, I'm not so easily scorned.
Annie: I'm the daughter of the kingdom's top knight & I have been trained to fight.
Annie shoves Artur aside and then proceeds to move deeper into the cave
Steiner: Annie! Where are you going?
Annie moves deeper into the cave, then turns around
Annie: Where I go doesn't concern the two of you.
Annie: When I get the Spirit Water, maybe you'll respect me then.
Steiner: We're in trouble, Artur! This place is dangerous. We must stop Annie.
Of course, trust this game to open the very first dungeon with DRAMA.
Heading deeper into the cave ourselves leads to more scripted fights, where Artur can still only choose between Attacking and Using Items. Not worth covering.
What is worth covering is the way this game does attacks and counter attacks.
You see, when you decide to attack a target, the game chooses between a regular attack or a critical hit. Occasionally, it will let you execute a second attack, which again can be a regular hit or a critical hit. So you have as much as four times the regular damage on one end, and damage worth one hit on the other. This leads to lots of variation in how effective the attack command is, which isn't really great for balance.
Now all of the stuff I said above for regular attacks? ALSO applies to counter attacks. Oh, and enemies get to use the same system for executing attacks, meaning you get to feel the brunt of the uneven damage yourself. It's honestly rather absurd, and should have really required some mechanism to control it, be it your to hit(which isn't a real stat in this but whatever), a timed minigame, or perhaps even a special command that consumed some kind of resource.
Technically speaking, there's the concept of timed hits alluded to in the manual. The problems with the implementation are that you only get the prompts for timed hits at random(a think sound when you or your enemies move to attack), and that getting the right inputs in doesn't really guarantee a double or critical hit or even a counterattack, merely raises the chances of it happening. So, uh yeah, basically it never matters much.

In addition to all of the above, Artur can randomly get a "dual attack" like this. Only he can ever score it, and it seems to either deal the damage of two dual attacks, or a critical attack. So effectively it's just an additional chance for him to score those two outcomes. Moving on.

The Tumblerabbit is as unremarkable as every enemy here, except for one thing; occasionally it jumps through the air to deliver an attack on Artur, complete with comical spring sound effect. It's quite hilarious actually.

Annie slowly steps backwards
Annie: Leave me alone!
Artur and Steiner cautiously move forward
Annie: I said, "Leave me alone!"
Annie runs to the stairwell on the left
Steiner: Wait. Which way was the dangerous path? It was... left. Right?
Indeed it was. So, they deliberately write Annie to be a brat and then deliberately also write her to be rash. I'm not sure we're supposed to care for her, outside of her being the protagonist's sister(or sister figure). Anyway, let's pretend we do care and go the dangerous way downwards.

Down here, a chest holds this Source of Wisdom. There's a bunch of various Sources around in this game, and they basically act as stat boosts. As you might guess, this one boosts the magic casting stat, called "IQ" in this game. This is perhaps the least useful stat to boost, and I'll show you why in due time.
At level 3, Artur learns FireLevel1. It's a decent enough spell for now, hitting about as hard as a critical attack. This also gives me an opportunity to showcase the uselessness of IQ. Pictured, Artur hurling a fireball without drinking the Source of Wisdom:

And with:

Admittedly, that's just one IQ boosting potion, however it will quickly become apparent that IQ has little to no impact on how much damage a spell does. Infact, it's best to consider spells as fixed damage sources. This also leads to spells becoming outdated surprisingly quickly, as you can imagine.
Unfortunately, for as helpful as FireLevel1 is now, Artur only has enough MP to cast it once. So the rest of the dungeon is still the same ol' Attack and Item routine.
A scream is heard as Artur descends further downward
Steiner: Artur! That's her! Something must have happened! Let's hurry up and find her!
Down the stairwell lies..

Ugh, that looks gruesome. Let's hurry up!
Steiner: Annie is in trouble! Artur, what are we going to do?
Artur and Steiner nod to each other, then proceed to move to both sides of the creature and fight it. Unfortunately this has little effect, and both are thrown back hard.
They try once more, and fail, this time running into each other. The creature moves away from them.Annie: HELP!!
Both of them get up, and Steiner tries to pull Annie out of the grasp of the creature, with little successAnnie: Steiner, please... Help me!
Steiner: Just hang on, Annie! I'll save you.
Annie: Steiner, I... I can... no longer...
The creature promptly slips out of sight, tossing Steiner backwards. When both of them get up, all that remains are a bunch of bubbles where the creature once rose from. Steiner checks the spot dejectedly, and this playsSteiner: Artur, we have got to do something! There must be a way to rescue her!
So we went from Annie being a brat and running rashly into dangerous caves to Annie being captured by a horrible swamp monster, possibly dead. That sure escalated quick.
Leaving makes Steiner prompt this:
Steiner: Artur, we can't leave her here! We must do something!
Indeed, we must.
Taking the other stairwell, we soon end up here:

That sure looks like a spring to me, so let's fill our Flask! To do this, we need to select the Flask in our Item menu while facing the spring. Using it gets us the Spirit Water, complete with happy jingle. Unfortunately,
we can't drink it, merely look at it.
Rushing through many, many dull encounters back to the swamp on the left...
Steiner: Artur, try everything you can to save Annie!
Using the Spirit Water on the swamp leads to...
The creature from earlier shrieks in pain, rising up and tossing out Annie in a fit.
It retreats quickly.
Steiner rushes to Annie's sideSteiner: Annie! Wake up! Wake up, Annie! It's no use... We have to try something else.
Artur nods and pours a few drops of Spirit Water into Annie's mouth.
Both of them then wait and watch as she slowly regains consciousness.Annie: Uuummm... <cough>
Annie: Uuuh... <cough>
Annie: Mmmm...
Annie gets up, everyone rejoices, and this tune playsAnnie: I... I was... ehh?
Annie looks around a bit perplexedAnnie: The Kraken dragged me underwater! I don't even want to think about it!
Steiner: Annie, I'm so relieved. You sound all right.
Steiner: Artur, let's go. We shouldn't stay here too long.
Annie: I'm scared... I can't walk alone. Take me with you, okay?
And with that, we
finally get a second party member. Took them long enough.
Annie is more mage like than Artur, as far as her stat distribution and equipment goes. She also starts at level 1 and currently hits for rather pathetic damage with her rod, making one wonder how she even got through the cave in the first place. While she starts with 10 MP, she bizarrely knows no spells to cast.
You might also be noticing a pattern with the dialogue.. a lot of it repeats a few key points over and over, and honestly leaves little room for interpretation. As such, I think it would be wiser for me to start summarizing scenes and only highlighting some notable sentences when necessary.

Having a second party member finally lets us access the Tactics menu. Charge is supposed to make the team fight aggressively, Fight make them fight normally, Reserve make them conserve resources like items and mana, and Guard make them defend themselves. Honestly, none of them matter though, since Manual lets you have direct control and is often better for decision making than the other options.
The only other thing of note in this cave is this bizarre room:

I have no clue if this is supposed to be us on a cliff top, or some otherworldly landscape in the depths of this cave, or perhaps a rendering error.
Well, this cave was pretty dull when we had one party member, and with two it's honestly not even worth mentioning. Time to leave!
Steiner: Artur, it's getting dark. We should hurry back home.
Will do, but that will have to wait for next time!