Whilst the preference of evolution would point towards a tool-using appendage being near the eyes and mouth (feeding yourself and at least seeing what you're doing) there are still possibilities for their evolution. The most likely scenario is one where they evolved to grip with their tails to climb/manage difficult terrain. On earth we have these tails in all sorts of animals, monkeys, marsupials, mice etc.
Based on earth creatures we see prehensile tails used for climbing, anchoring in bad weather and carrying. We can assume our creatures evolved their tail for other uses and then, as they became more sentient they realized the tail could be used for other things.
To your question:
What kinds of ranged and melee weapons can they use with their tails? What kinds of armor? what kinds of war tactics and defensive fortifications?
So we don't have just one question here but if we decide on weapons that would be used we can narrow down the others.
I would imagine the long tail is useful for projectiles as some form of Atlatl to provide more power and stability to the spear. Though here we should note that the whole body needs to be used, it is unlikely the spear could be thrown any distance from tail strength alone. I imagine the spear would be held in the tail and then, after a couple of bounds, the front half would bear down and the back end buck upwards, providing extra power to the throw.
The problem we reach with melee weapons is the fact that the tail is unlikely to be well protected, it is mostly muscle which can be cut fairly easily. Armouring the tail would reduce its flexibility drastically and it is unlikely to have enough strength for iron-age armour to be practical protection whilst keeping the tails useful. If your opponent holds their weapons in their tail and you focus your blows on those you could well sever the tails and leave them, mostly, harmless. Training in using a melee weapon in your tail would not lend itself well to holding a weapon in your paws.
It could well be that it pays the front-lines to already have their tails cut off and cauterized before any battles to remove the easy target that could result in them bleeding out on the battle field. This would take some getting used to but they could still balance okay with practice - perhaps something which is done to younger potential soldiers.
Long flexible melee weapons would suit tails best so things like whips, light maces and such. Swords would be difficult to support against a parry. As such the body of the front lines should be well armoured and their tails (if you want to keep them) should hold such weapons.
Without the tail you could have blades attached to the armour on the front legs (since the paws aren't suited for grip). That way the body can support the blows a little better.
- Aim for the tail if your enemy has one.
- Light armour keeps the flexibility and graceful motion that is often seen in cats - this would probably be leather (a potential cover for the tail).
- Throwing spears - this would need to be practiced a lot to build up the strength in the tail but bear in mind the muscle is heavy too and with no real bone structure to support it some strength would have to go into not dragging the tail along the ground.
- Front line melee should aim mostly to hold back the enemy from the spear throwers behind.
- Some could, perhaps, mount weapons on their backs and use their tails to operate these. For example a cross-bow attached to the back (and avoiding the head) could be drawn back by the one tail and loaded with the other for quick firing.
In the end, though, there are many possibilities and the only limit is your imagination.