As for increasing the HP/defense/damage of things in siege that's just a bandaid, good till nect level cap increase, every time they raise level cap they'll have to do it all over again. Each time they raise the hp/defense/damage on them, it raises the minimum level of players who can be effective in siege. If the minimum effective level for players in siege is too high there won't be many new people joining.
For most sieges anyone below 60 is just a farmer or a distraction, not much reason to join if you need to be 70+ to do more than farm
All pretty much true. I have a 69/62 R/M that is reasonably effective, and spends much of his time farming merits (middle area, and is now able to farm tower guards fairly well.) His gear is a mixture of level 55, 60, and 62 items, decently statted. I'm working on getting better stuff for him and, obviously, leveling him even further.
Awhile back, when he was level 64, he was completely unable to kill or even damage defensive towers or items such as treaches or eyes. But once he got to level 68 or 69, he was much better able to damage/kill those defenses. So yes, character level *does* matter, along with upgraded skills. (His main offensive skills are at least level 60--there's just not enough TP available to raise them much more than about 9 levels below his current class level.)
Increasing the amount of defense/damage on defensive items (again), or increasing the level of the tower guards (again) would, as you've said, make lower level characters such as mine less and less useful in siege. It's hard enough to damage or kill a character that's 10 levels above your own, if it's reasonably statted for *it's* level. And I agree that newer characters (not necessarily newer *players* might well be reluctant to join in Siege if they'll be relegated to "support", only being able to farm balloons or tower crystals at the castle. While very useful, there are a limited number of balloons available, and it's my opinion that it's better for one player to work a given side of the castle in order to collect merits and pass them along or set up defenses.
Those defenses may be "1-shottable" by relatively uber-geared players, but they're also valuable in slowing them down until the cavalry arrives to shore up the defense. Sure, it's a bit disenheartening if you've spent time, merits, and crystals to set up those defenses, only to see them taken out. On the other hand, they *do* slow down the opposition and, outside of Fearless or other immunes, Fkame Towers *can* Fear the attackers, while Electric Towers have a chance to disrupt a spellcaster and pop Rogues out of Hide so *they* can be targeted and dealt with.
Leave them as they are for the time being. Bog knows RW left the tower guards and defenses at level 55 for more than two years. They don't need another upgrade quite yet.