From "Extending Pack Capacity (On The Side): Part 2: The Solution"
Section: Design - Access
The Tower features a full length vertical zip opening housed underneath a peaked "waterproof cap" overhang. We chose YKK Vislon zippers. These are superb non-snagging zips with an easy glide and provide a very good water-resistant seal being plastic moulded.
The lack of quick and easy access to kit was at the root of our dissatisfaction with many of the current products on the market. Even the best of them in terms of access, the Bach XL Side Pocket is not perfect, due to its use of a storm flap. Remember "the notes you don't play", well this was one of those notes. We thought long and hard about zip storm protection. The problem with storm flaps is that although they look nice, tidy and protective, in practice they can, a) act as a gully directing water into the zip housing (since these things stand vertically), and b) when you're wearing winter mitts and it's dark, it can be a faff trying to locate the zip pull which has lodged under the storm flap. We chose to forego this approach and instead designed a rather novel "cap" for the Tower. This also acts as a "pull-tab" when opening the zip, but is much wider than most and extends further, acting as a brim, so run-off from the roof of the Tower is pushed away from the zip below.
Because we've made the Tower almost symmetrical, it's agnostic about which way it's facing. So for example, you can have the zip facing forward (where the user can potentially reach over their shoulder and access kit at the top) it can face out to the side (its default position) or face backward (useful if walking into a storm). However, what we've found most useful is the ability to turn the Tower zip facing the pack - obviously this completely negates access, but sometimes that's a good thing. For example when in transit or in urban environments where "pick-pockets operate in this area" it's sometimes helpful to make access as difficult as possible. Also, in extreme storm conditions, where the thought of taking a relaxing break is the last thing on your mind, this setup also provides the most extreme level of weather protection, making the Tower practically 100% water-proof.