Recently I've been just putting archers in my fortress because of the x4 attack, but would having some cavalry in there help more then just all archers? And if so, what would be the cavalry to archer ratio?
If you have an army in the field you want the cavalry to be with the army. That way you have flank cavalry. If you do not have a field army the cavalry in the fort might be a good idea.
In order to out shoot heavy archers in a boxed mountain an attacker would need 4X4X1.2*1.25= 24 times as many archers. With 266 archers (s)he has to send 6400 heavy archers. Not likely. I am also not sure if the 4X rule applies to the archers. If it does, then an attacker could use 6400 archers only if (s)he used 0 swords. If you have cavalry in the fort you still win the moral even if the attacker has obscene numbers of archers because the defending cavalry have more hit points than the attacking archers.
2000 heavy cavalry with 400 archers will buy you several extra rounds of combat. The trade off is that the cavalry increases your networth and pushes you in range of more attackers. The cavalry would buy you one extra round of combat. Could be more than one if the attacker is pushing 50 troops per catapult.
The answer to the original question is no. You cannot be guaranteed to be able to defend. For any set up of research, buildings and troops I can put together a set up of research buildings and troops that can break through without dropping below 50 moral and is well withing 3X range. Usually within 2X. On the other hand, players who have a heavy siege army are much more vulnerable to attackers that are 4X to 6X.
If you did lose a large pile to a empire with 2.5X your networth he/she will be out of range when the money is spent. Players who have maximum siege are rare.