Twincast makes Hemah summon two Water elementals instead of one.
Water elementals split into two (arguably weaker) water elementals when they die.
Summoning trait makes summons last three turns instead of one, making it possible to have 6 large water ele's/12 'split once' water ele's/ 24 'split twice' water ele's, afaik.
EDIT: The Tower adds +1 strength to the elementals, iirc.
Well, Summoner now only gives 2 turns instead of 3. But the real kicker is the Tower. Normal Water elementals split when they die into two Weak elementals, and then if those die nothing happens. But Strong Water elementals (which you only get from the Tower of Elements) split into normal elementals when they die, which can then split AGAIN. Twincast also doubles everything.
So with the Tower, a properly promoted Hemah would summon two Strong elementals, which would attack and die, giving four normal elementals, which attack and die, giving eight Weak elementals. 2+4+8 = 14 attacks.
Summoner doubles things again. If you let Hemah summon two Strong elementals, wait a turn, and let him summon two more, then you could have as many as 4+8+16 = 28 attacks.
If you're Keelyn, things get silly, because puppets multiply the effect of Summoner by a factor of 4. Hemah summons two puppets, which each have twincast. Everyone waits a turn (the puppets don't die because of summoner). Next turn Hemah summons two more puppets, giving a total of four puppets. Each puppet summons two Strong Water elementals, giving 8 elementals summoned this turn. Everyone waits one more turn. Two puppets die, and Hemah summons two more to replace them, giving a total of four puppets again, who summon eight more Water elementals. So there are a total of 16 Water elementals (8 this turn + 8 last turn) which can attack this turn. Assuming they all attack and die (16 attacks), and all the normal ones attack and die (32 attacks), and all the Weak ones attack (64 attacks), that gives you 108 attackers produced just from Hemah, given a 2-turn build up. I think that would be enough to crack just about any city.