¿Qué significa "And we must unite inside her" ?

In Harry Potter and the Order of the Phoenix when the sorting hat warns the school about upcoming dangers in it's song it says "And we must unite inside her"
The song is as follows

In times of old when I was new

And Hogwarts barely started

The founders of our noble school

Thought never to be parted:

United by a common goal,

They had the selfsame yearning,

To make the world's best magic school

And pass along their learning.

'Together we will build and teach!'

The four good friends decided

And never did they dream that they

Might some day be divided,

For were there such friends anywhere

As Slytherin and Gryffindor?

Unless it was the second pair

Of Hufflepuff and Ravenclaw?

So how could it have gone so wrong?

How could such friendships fail?

Why, I was there and so can tell

The whole sad, sorry tale.

Said Slytherin, 'We'll teach just those

Whose ancestry is purest.'

Said Ravenclaw, 'We'll teach those whose

Intelligence is surest. '

Said Gryffindor, 'We'll teach all those

With brave deeds to their name, '

Said Hufflepuff, 'I'll teach the lot,

And treat them just the same. '

These differences caused little strife

When first they came to light,

For each of the four founders had

A house in which they might

Take only those they wanted, so,

For instance, Slytherin

Took only pure-blood wizards

Of great cunning, just like him,

And only those of sharpest mind

Were taught by Ravenclaw

While the bravest and the boldest

Went to daring Gryffindor.

Good Hufflepuff, she took the rest,

And taught them all she knew,

Thus the houses and their founders

Retained friendships firm and true.

So Hogwarts worked in harmony

For several happy years,

But then discord crept among us

Feeding on our faults and fears.

The houses that, like pillars four,

Had once held up our school,

Now turned upon each other and,

Divided, sought to rule.

And for a while it seemed the school

Must meet an early end,

What with duelling and with fighting

And the clash of friend on friend

And at last there came c morning

When old Slytherin departed

And though the fighting then died out

He left us quite downhearted.

And never since the founders four

Were whittled down to three

Have the houses been united

As they once were meant to be.

And now the Sorting Hat is here

And you all know the score:

I sort you into houses

Because that is what I'm for,

But this year I'll go further,

Listen closely to my song:

Though condemned I am to split you

Still I worry that it's wrong,

Though I must fulfil my duty

And must quarter every year

Still I wonder whether Sorting

May not bring the end I fear.

Oh, know the perils, read the signs,

The warning history shows,

For our Hogwarts is in danger

From external, deadly foes

And we must unite inside her

Or we'll crumble from within

I have told you, I have warned you ...