I did a bit of testing, and I think I can provide some feedback about creature strength, and the ways how I would adjust some statistics (adjust can be done in several ways, units can be adjusted to be offensive or defensive, in many cases I made choice based on current stats)
Generally many units in this town are very strong, with fairly high price. You can adjust units to be worth their price but keeping all units “fairly strong” results in town with too good units and if player can afford the gold then the town makes player able to get stronger army than the other.
About particular units:
level 1 not-upgraded: I would keep it as it is for now, maybe add +1 attack, seems to be average unit
level 1 upgraded: Not overpowered, but extremely strong. 2-4 damage, 8 hitpoints and high speed. I think I would increase the cost to 90 and keep it as it is or decrease damage to “2-3” and add +1 attack point. If you do not want it to be another pikeman-like unit you can keep downgrading stats and cost
Generally level 1 abyss units should not have base growth bigger than 14 with their strength and cost
level 2 not-upgraded: Seems to be ok cost and balance-wise. Similar unit to dwarves.
level 2 upgraded: Seems to be a bit too overpowered with their damage reduction, 18 hit points and 2-5 damage - most among all level 2 units. The most intuitive adjust seems to be damage downgrade to 2-4, this unit has already huge defensive capabilities. I would also increase the price to 165, because of the damage reduction ability, even though dwarves have 20 hit points and 1 more attack and defense point and they cost 150.
Growth seems to be good. 8-9 base growth is safe range for this unit.
level 3 not-upgraded: pretty expensive units, average hitpoints, high speed, rest of stats very strong. I would add -2 hitpoints, -1 attack and defense, -1 max damage to make this unit more “average”, unless you plan to nerf other units more.
level 3 upgraded: same as not upgraded, speed of serpent fly, stats of griffins (or whatever their name is). And I would do same as for not upgraded: get current creature stats and add -1 attack and defense, -1 max damage, -2 hitpoints
Growth looks ok.
level 4 (both levels): Generally this unit does not need changes I think, weaker stats than mages / archmages, but lower cost, and can shoot while enemy blocks unit.
level 5: Hard to say, stats are rather good. I would not decrease cost for this unit.
level 5 upgraded: Here comes the problem, this town has already many good units and this one together with seahorses looks like best candidates to leave not too strong. 70 hitpoints is huge amount, and rest of stats is pretty good too. I would keep hitpoints at 50, maybe 55, and maybe give additional +1 attack point and keep the cost as it is. The unit can already hit all nearby enemies.
level 6: This is interesting unit. 16-32 damage sounds like another “original” value good to beat new unit level stat records but this unit seems to be interesting when it stays strong. I would give -1 attack and defense point and change cost to 1050.
level 6 upgraded: I would give -1 attack and -2 defense and keep rest as it is. Can also give 25% shooting damage reduction and increase cost to 1400.
level 7: This unit has pretty weird balance in my opinion. It has same damage range as giant (also same average damage as angel), and 20 attack and defense, what seems to be unnecessary, this unit can be strong if adjusted down and whole town becomes less likely to be overpowered. I need to consider that unit is pretty slow, my personal preference would be to make this unit more attack-oriented I would give it 20 attack and 17 defense, 40-55 damage and keep rest as it is.
level 7 upgraded: Here comes unusual stat change: damage gets boosted over value of any other unit, hitpoints become very high, but attack and defense stay at poor value. If the unit cost is not meant to increase then I would make damage 45-55, 24 attack and 21 defense with keeping the rest as it is
Another things I noticed:
“Deep ones” fight value is way too high
unit buildings take too much wood
resource silo resource does not match 7th level creature building & upgrade resource