The architecture was mainly made of Mudbrick-on-timber buildings. Very few people actually lived inside the city itself, which makes estimating populations difficult. Inside the city there where mostly granaries, workshops, temples and other social areas. The walls would have impressed any tourist or visitor of the time, the walls would have been made of roughly hewn stone for 3-4 meters, after which mudbrick covered the rest with up-to maybe 8 meter total height. Separating the lower and upper cities where the postern wall which would have been 71 meters long. The point of the postern wall is not known. When defending Hattusa the gates would've been it's weakest points. Hattusa would have encompassed an area of 1.81 sq.km.

Hattusa was actually destroyed by Anitta[en.wikipedia.org] in the 18th century BC, when the dynasty was still based in Kussara and Hattusa was foreign territory. With the city being revived and reoccupied during the 17th century BC and it became the capital of Hatti.

Fun fact, Kanesh, in-game is one place where the Hittites originated from, from where Hatti may have actually started. Kussara is technically the original point but Kanesh is where they grew powerful. Kanesh was the early tradehub of the Kingdom before Hattusha was re-occupied by the Neshites/Hittites and became the capital.

Acropolis
Also called the Buyukkale today. This would have been where the Palace was and it was also fortified. When you deal with Suppiluliuma II in-game, this is where all that diplomatic magic happens. Dignitaries where hosted here. The King and many of the dynasty's cadet branche members probably lived here. To the north-west of the Acropolis is the Hittite worlds greatest complex and biggest temple, the Temple of the Storm God[en.wikipedia.org].
It contained two shrines in it's innermost which was unusual, since most temples contained just one. It is thought that these where dedicated to the Storm God Tarhunt/Teshub and the other to the Sun Goddess of Arinna/Hebat. This is probably where you would go do many of your religious activities in-game. Granaries where located in the Lower City, directly north of the Acropolis behind the Buyukaya gate and well as between the postern wall and Temple of the Storm God. These graneries would have been dug into the ground and then covered and checked regularly, the grain weren't left in open fields.

Buyukaya gate & Postern Wall
The postern wall is the above mentioned wall that separates the both halves of the City. While the Buyukaya gate is the one with quick access to grain stores. There where 12 rectangular grain stores in this district. Hattusa and it's food stores where used to supply the Hittite homeland. Which made the city strategic just for that fact alone.

Lions gate
This gate would've been the main entrance into the city. This is where most people would enter the city, there would have been guards and potentially even some checks or tolls.

Sphinx Gate
This game woud've been the first gate you'd see when arriving, but not main gate into the city. It was used during religious ceremonies and otherwise closed. Though this is not how the gate would have looked like, there should have been a massive mound and a too narrow of a passage (for tw in-game) terms.

Warrior gods gate
This would've been the 'backup' gate, if the government needed to open a second gate for traffic this was usually the one used. Alternatively it has also been proposed it may have been a ceremonial exit gate.

In the upper city itself the would've many more smaller temples. With the Main Temple Quarter being the centre of the city.

Book source Warriors of Anatolia by Trevor Bryce
Another book source is The Hittites by Damien Stone, similar to that of Trevor Bryce
Wikimedia pictures
A set of reconstructed walls presently: https://upload.wikimedia.org/wikipedia/commons/4/41/Hattusa_reconstructed_wall.JPG
Also used: https://en.wikipedia.org/wiki/Hattusa

Check also out the overall Historical Guide to TW Pharaoh by Saint Scylla

Tips: If you think you are a hardcore history nerd then you can view this lecture
https://youtu.be/WWLFpnxtJ8c?si=XQUCKa2k9QqHE8XF
While Kings and Generals made a great 3D introduction into the city
https://youtu.be/CgH4CxQrgRc?si=CCdgK-G1x5VKFrDy