I'm looking at Wizard's Tower and then looking at other familiars and wondering why it's so cheap.

Fellella, Pixie Familiar - no armour, 6 health, 1 channelling, casts only Enchantments - Costs 12
Sersiryx, Imp Familiar - 2 armour, 9 health, 1 channelling, casts only Level 1-2 Fire attacks or curses. Costs 12

Wizards Tower - 3 armour, 7 health, 1 channelling, non epic attack spells - costs 7?

If I look at that I can see that different cards have relative strengths and weaknesses..

The creature familiars can be buffed (but at a mana and action cost)
The creature familiars can move.
The creature familiars have instrinsic attacks.
Sersirxy has Flame -2, Fellella has a rather nifty 6+ defense (unlimited uses)

But the Wizard's tower has spellbind so can spam, while still changing spells freely
Has a hefty 3 armour
Has, effectively, it's own Quickcast marker (the ready marker)
Range 0-1, meaning it can be easily played in a central zone for almost full board coverage with it's spells.
Is a Quick spell, not full.

If you look at other Quickcast familiars - Sectarus the Rune Sword can only cast curses and only after the Mage hits something with it... no actions of its own, vulnerable to dissolve, no spellbind, no range on its spells - Costs 10.

And comparing the Wizard's tower to a Thoughtspore (1/0/7, no spell change all game, cost makes the Thoughtspore sad.

So the Wizard's tower has major strengths, and minor weaknesses compared to other familiars, all at a far reduced cost.

Based on its peers, I would be costing it around 13-14 minimum and Epic.