well for example, I've been accidently camping in EMA for about a year now, and I mostly manual battle. I have a few watch towers but that's about it. I think it amounts to 30% before a potion (there seems to be only 1 type of attack potion). ANd I would think it would equalize with ages because you generally only get to fight the same age no matter where you do it.
A few things then:
1) A watch tower is in GBG not a players city. In GBG that does not add to your attack or your defense it has to do with the chance that you would increase your attrition. Do you mean watchfire? If that is the case it gives a defense boost to your army defending your city (not when you attack).
2) There are actually three types of attack boosts from potions: 20%, 10%, and 5%. The thing is those boosts are on the base attack number of the unit you are using (it doesn't take into consideration any other boosts). So if your unit has an attack of 50 and you use a 10% potion it would have an attack of 55.
3) Attack boosts can be very useful so generally speaking the more the better (it's not quite that simple but we can get into that later). You want to be able to kill the enemy units with 2 hits that's the advantage of better attack. It gives you a better chance of killing the enemy in 2 hits and a dead enemy doesn't shoot back (whether it's manual or autobattle). Note: there is a rather large great building (Arctic Orangery) that can sometimes turn that into a 1 hit kill but it may not be of a lot of value to you at the moment (again we can discuss that after a bit more information comes out). One trick to boost your attack power quickly besides the potion is to use tavern silver to get another boost. So if you already have your zues and COA to level 10 (30% boost apiece), use an attack potion of 20% and the tavern silver your hypothetical attack number would be base 50 + 15 (zues) + 15 (COA) + 15 (tavern) + 10 (attack potion 20%) = 105 attack.
4) Another factor in battles is your defense boost while attacking. Note that this is different from the defense boost for your defending army. You need to make sure you are placing the right Great Buildings and other buildings to get the boost that you want to get for your game purposes. So in this case with the same factors in #3 above and a theoretical defense rating of 50 your defense score while attacking would be 50 + 15 (zues) +15 (COA) +0 (attack potion doesn't add to the defense of your attacking army) + 0 (tavern attack boost doesn't add to your defense of your attacking army for an defense score = 80. This score will determine how much damage your army takes while fighting. Generally speaking attack is easier to get so it's better to grow that 1st so you can reliably get 2 shot kills (or better) and then work on your defense score as this will help lessen your loss of troops.
5) While manual battling are you on PC? If so you can see the turn order of the enemy units which lets you pick which enemy units to kill. In EMA it's generally better to kill a unit before it can fire as that will lessen the damage you take and give you an advantage. We can go into this more as a separate question if you want more details.
6) You can actually proceed on the Continental Map as far as you can either fight or negotiate so you can actually face troops of higher ages. You may not want to advance to far without doing some research because if you are careful you can get some unattached troops of higher ages which can be useful for you.
7) Do you have any rogue hideouts? Rogues take a day to train but they can be very useful in fighting.
8) GE while you don't face higher age troops the higher the level the higher the enemy attack/defense boost is for that encounter. As you age up the enemy GE boosts increase. GBG you never face higher aged troops but each battle you add to your attrition (assuming your guild hasn't built enough camps and watchtowers to make the fighting attritionless) so you'll want better attack and defense so that you can fight higher and higher. Eventually if you get your boosts high enough you can autobattle and get more battles in on GBG or at least faster battles.
Let me know if you have follow up questions.