Skulls are the collectables that are somewhat harder to find in Halo Infinite and one of the first ones can be find in The Tower, or rather - on it.
Hunting a douchebag that tortured a Spartan is one of your first few missions in Halo Infinite and he's located at The Tower. The area contains several collectables, one of which is a skull that can increase the chances of hearing rare dialogue while slapping Banished cheeks all over the Zeta Halo.
You don't actually need to complete the mission in order to collect the skull since it's not locked behind anything that would be only unlocked through progressing objectives.
Instead, the skull is located on the roof of the Tower, which can only be accessed from the outside. That said, you will need some Spiderman-like skills to climb to the top and I would highly recommend upgrading your Grappleshot to Tier 3, where the cooldown is reduced by 40 per cent.
Once all that is done, simply keep climbing the walls of the Tower with the grapple. Alternatively, you can take the angled pillars halfway up to the top and roughly three swings from the grapple will do the rest of the job.
Once you're on top of the tower, it's easy to spot the skull on the platform that has a bunch of something that resembles memorabilia. This is also the place where you can find the Craig Easter Egg so make sure you check that out too.