Why didn't my apples caramelise?

Today I tried to make a Tarte Tatin, with mixed results. Everything tasted great, expect that my apples didn't really caramelise.
Note: This recipe is more like an interpretation of a real Tarte Tatin.

As instructed, I first made caramel (sugar & water), which I then is poured onto a plate to cool and harden, just like the recipe said.

In the meantime, I peeled four apples (Boskop), then cut them into quarters and removed the core.
To prevent the apples from browning, I put them in a bowl of water with lemon juice (this was not in the recipe).

After all four apples were ready, I put the quarters into an oven-proof pan after washing them & drying them with a paper towel (to remove the lemon juice taste).

Following the recipe, I broke the caramel into small pieces, put them on top of the apples, squeezed half a orange over the pan and added a few tiny bits of butter. Then I covered the pan with aluminum foil and put it into the oven at 180°C.

As the recipe did not tell me what type of heat to use, I used top & bottom heat.

According to the recipe, the apples would be caramelized after 20 minutes. When I looked at them after 20 minutes, the caramel didn't even begin to melt, so I put them in for another 15 minutes. After that, the apples basically turned into mush, like they had exploded.

My guess is that by putting them into the bowl with lemon water, they probably sokaed a bit, which means they were moister than usual.

Also, it sounds a bit strange to me to first make the caramel, letting it harden, just to reheat it again – that simply did not work as expected, maybe because it was to thick or the pieces were too large.

How can I avoid that my apples turn into overly soft, shapeless apple things?
Is there a better way to caramelize apples, maybe a way that keeps a bit bite to them?
Did you notice anything wrong in my process?