What does it mean

Question

400年以上前の白い壁が残っていて、建物の形が白鷺という白島が羽を広げて休んでいるように見えるので しらさぎしょう (I'm not sue if that's correct spelling) とも呼ばれていま.

It's from the book TOBIRA. Can anyone help me break this sentence down to smaller, more comprehensible bits?

Ok, so as far as my understanding goes it means that:

There is a white wall that is more than 400 years old and a white building that looks as heron that spreads its wings and rest and that's why it is called "shirasagishyou"

It's about Himeji Castle.
BUT This sentence is sooo long I'm not sure what kind of grammar is there anymore. So many ga particles.

建物の形が白鷺という白島が

Why "ga" after "kata" then "toiu" and then another "ga" after this second "ga"? It's pretty straightforward. Can anyone help?

Answer 1

You can break this sentence in two:

Sentence 1: 400年以上前の白い壁が残っています

Sentence 2: 建物の形が白鷺という白鳥が羽を広げて休んでいるように見えるので白鷺城【しらさぎじょう】とも呼ばれています。

You got the first sentence, so I'll skip the explanation for sentence 1.

So, let's look at the very base part of the sentence 2.

The base sentence is

白鷺城と呼ばれています

ok, now, you want to add the reason why it's called "Shirasagi-jou". That would be...

建物の形が_____のように見えるので、白鷺城と呼ばれています。

It's called "Shirasagi-jou" because the shape of the building looks like _____.

now, how does the shape of the building look like? If it looks like a swan, you could say 建物の形が白鳥のように見えるので..., but that's not exactly what you want to say. The shape doesn't look like ANY SHAPE of swan, it has to be like a swan that is spreading its wings and resting, which is 白鳥が羽を広げて休んでいる

So, you now have

建物の形が　白鳥が羽を広げて休んでいる　ように見えるので、白鷺城と呼ばれています

Now this is still not exactly what you want to say because the shape is not ANY swan spreading wings and resting. It has to be a certain kind of swan that is called Shirasagi, which is 白鷺という白鳥

You'll add this information right before the 白鳥 and you get:

建物の形が、白鷺という白鳥が羽を広げて休んでいるように見えるので、白鷺城とも呼ばれています