Late start today, but the sky is mostly blue and so I'm headed to the fortress to check it out. I've got my repaired telephoto lens along and we'll see what I can take pictures of from the high vantage of the fortress. I haven't really had a chance to give it a proper testing out since getting it back from the repair shop in Belgrade. I did take it with me yesterday and got some shots of birds in flight Danube and the focusing seemed to be crisp and rapid enough to handle it.
There is a free market in the city center that has a bit of the aspect of a neighborhood garage sale, with lots of used household wares and used children's clothing and such for sale. I think anybody can just grab a free space and sell whatever. One person was selling nothing but cacti.
I'm having a lunch consisting of a couple of bread rolls shoved into my maw with some water from my water bottle to wash it down. Meeting up with my father next month in Istanbul is going to be expensive and I might have him bring me a laptop.
Day one of exploring the Petrovaradin star fort. The place is enormous and I was very much right about this being a multi-day exploration. I explored the lower sections down by the waterfront and then moved up into the fortress via a a four gate system and then explored the Southern of the two major sections of Bastions, Ravelins and Caponiers.
There's a huge network of brick tunnels inside all of these structures and some of them are accessible, mostly because there are so many entrances that it's hard to secure all of them properly and they just don't have the budget. The northern lobe which is closer to the bridge and city is kept comparatively neat and presentable for tourists, but the equally large southern complex is largely overgrown and untended.
I walked around in a few with my flashlight out and made sure to keep well oriented. Many are on multiple levels and there are vertical traverses in places between the levels, so there is a fall risk in some of these in addition to the risk of collapsing brickwork, which is several hundred years old at this point. There is some sort of official "catacombs" tour associated with the northern part of the complex, as well as a major on-site museum.

I'll probably do the northern part as well while I'm here, even if I feel like I've already gotten a since the place and its design. I'll have to see what my body is up for tomorrow. It was very hot out today and I went through a lot of water. I think I might be a little into sunburn/heat stroke territory. Did 16,000 steps for the day.

Stopped at a bakery about a block away from my apartment and picked up two really big chocolate croissants and 10 Mantije balls for breakfast for 400 RSD.

Here is the fortress as you encounter it walking over the bridge from Novi Sad. Looks impressive, right? But this is actually a pretty small chunk of it.
This is what the Petrovaradin fortress looking like from space:

The part you're seeing from the bridge corresponds roughly to the bits outlined in blue here:

You're seeing nothing of the landward defenses at all, where you've got layers up layers of bastions and other defenses.

So, why all those layers and layers of defenses? In short, the Ottomans.

The Austrians had captured Petrovaradin from the Ottomans in 1687 during the Great Turkish War and promptly began drawing up plans for a more modern fortification, as what they'd taken over from the Ottomans was laughably basic by the moderns standards of the warfare of the day.

It looked something like this at the time.

That diagram is barely a doodle compared to this exquisite piece of military draftsmanship which replaced it.
In 1694, the Ottoman's attempted a siege on the fortress, which I imagine was at somewhat of a preliminary stage at that point. They besieged it for 3 weeks but had to turn back to Belgrade, their attempt a failure. Further Austrian victories in the area lead to a peace being concluded in 1699, but it didn't last long and in 1716, the Ottomans attacked, having gathered a force of 150,000 troops at Belgrade and marching on Petrovaradin, which is just some 40 odd miles away.
The Austrians managed to cobble together ~75k troops, only half what the Ottomans had in the field, but used the fortress to their advantage and won a great victory under its guns. The Ottomans lost ~20k people including the Sultan's son-in-law, who was in charge of the army, as well as losing over 170 cannons and the Ottoman war chests (payroll for the armies). Within a year, the Austrians were occupying Belgrade.
The Austrians would continue to fortify Petrovaradin throughout the 18th century, with major new construction in the 1750's. Some 10 miles of tunnels and counter-mine systems were built throughout the place. The end result is something like what you see today, which is sometimes referred to as the Gibraltar of the Danube.

Here are my explorations of the fort as I encountered things.

This fortress clock overlooks the town and can be easily seen from across the river. An interesting feature is the hands are reversed, with the large/long hand pointing to the hour and the short hand to the minutes. This was to make the clock more easily readable to fisherman at a distance.

In addition to the fortifications on the heights, there are actually some pretty substantial fortifications right along the river, to provide additional defense against ships and against any forces which might be landed below the fort's guns.
Access to these was closed off pretty well, but you can poke your camera in through the bars to see inside.
I walked along a road which runs beneath the cliffs on which the upper fortress stands and above the lower bank of fortifications which fronts the river. There are some old paths leading up the cliffs to postern gates, but good luck assaulting the fortress that way.
Seriously, good luck with that.
I waved up at people high up on the walls a few times.
Continuing along the road, you get the first sense of the nested layers of bastions and ditches which form the fort's defenses. Take one layer and you're just looking at yet another layer.
I finally came to a gate, an actual way into the fortress, the first actual way in other than the postern atop the cliffs, which was almost certainly locked.
@Infoseepage I am so enjoying this walk with you! I’m gonna go work in my gardens, and cannot wait to see the rest when I return. Thank you so much for all the detail and commentary. I can almost smell the sea air and feel the sun. 🥰