The only factor is whether or not the structure has a roof (a thatch roof, floor tiles don't count). Put a roof over the tower and it won't deteriorate.

My take on this would be to add a roof abover or underneath the platform where you stand on, so if at all, only the top floor will get damaged. There are other ways tho, like Placing the X-pieces into the walls I think. Or you could use the stake walls, they won't take damage from rain and have more health.
The version without a roof might be better for some people since roofing can easily block your sight, at least that's my experience.

Wood needs a roof. Stone not. So maybe a Stonetower is an option.

Weather will not degrade it more than 50%. You could just leave it like that, but keep the durability in mind if enemies start attacking it.

Originally posted by Danson:

The only factor is whether or not the structure has a roof (a thatch roof, floor tiles don't count). Put a roof over the tower and it won't deteriorate.

Angled wood pieces (the Xs and the triangle pieces) and stone floor do actually provide protection from rain for stuff below them.

Roof tiles are definitely the easiest bet though, you are correct on that.

Originally posted by shyPunK:

I have created simple tower to allowo me to shoot over walls at mobs attacking my base. All components of the tower degrade over time and have to be continually repaired. Is there any way I can stop this? When I use the same components in buildings they do not deteriorate. Do I have to construct the towers in a similar manner, with wall between the 2m vertical poles?

All help gratefully received, sPK

Normal wood degrades to roughly 50% but.... Have a look here.

https://steamcommunity.com/sharedfiles/filedetails/?id=2753983003

Where tarred wood items clip into normal beams they are "water proofed" just like the decorative beams.

I hope this helps your imagination and problem.