From Hindu Temples – What happened to Them
Well, the only liberality I took is appending the original poem quoted by Nainasiri in full from the book Munhata Nainsiri Khyat.

The Note from the Government of Gujarat says that the Rudramahalaya was built by Siddharaja Jayasimha in the 12th century and that it had eleven shrines dedicated to Akadasa Rudras. The Report of the Minorities Commission repeats this description with the elucidation that in the centre of this complex was situated the temple and in and around the courtyard were 11 other shrines dedicated to the Rudras. Both of them say that the temple was profusely sculptured and ornamented. But none of them mentions what has survived of the central temple or the surrounding shrines.

B.L. Nagarch gives greater details in his aforementioned article. He writes:

In about AD 944 Mularaja had founded the Rudra Mahalaya, but as he had to remain busy in invasions and other engagements he could not complete it. This temple fell into ruins during the following centuries.  Siddharaja Jayasimha took up the work of reconstruction of this temple on a scale greater than that originally conceived and could not finish the work till his death in AD 1143.

Rudramahalaya is the grandest and the most imposing conception of a temple dedicated to Siva. Only a few fragments of the mighty shrine now survive, namely, four pillars in the north and five in the eastern side, porches of the three storeyed mandapa. Four pillars in the back of it, a torana and a cell at the back remain in situ after being dismantled in the 13th century AD. With its adjacent shrines, possibly eleven, part of which was converted into Jami mosque later in the Mughal period, it must have formed part of a grand conception dedicated to Ekadasa Rudras.

Originally it covered an area of 100 x 66 mtrs. The central building itself occupies an area of about 50 x 33 mtrs. The mighty pillars of this temple are the tallest so far known in Gujarat.

It is difficult to visualize what the Rudramahalaya looked like when it stood intact and in all its majesty. No other edifice of a similar conception has survived. We have only some legendary accounts, one of which is from Nainasi who tells us how the Rudramahalaya was conceived and constructed. We give below a summary of what he has written at length.
Sidharao, says Nainas, saw the Earth in a dream, appearing in the form of a damsel and demanding that she be decorated with a choice ornament. The king consulted the learned men who could divine dreams and they told him that the ornament for the Earth could mean only a magnificent temple. So the king invited architects from every land and they presented to him models of what they could conceive to be the best. But no model satisfied Sidharao and he became despondent. At that time there were two notorious thieves in his kingdom, Khapra and Kala. As they started gambling on the Divali day, Khapra wagered that he would give Kodidhaja, the renowned steed of Sidharao, if he lost the game. He lost and promised to the winner that he would procure the steed by the time of the next Divali day. He wormed himself into the confidence of Sidharao, first as a sweeper in the royal stable and then as a syce of Kodidhaja. The king who visited the stable everyday was very much pleased with Khapra’s services and spent some time talking to him. One day the king confided to Khapra his (the king’s) disappointment in the matter of a suitable temple. Soon after, the thief ran away with the horse and stopped for rest only when he reached the valley of Mount Abu. All of a sudden he saw the earth split and a temple came out. Gods and Goddesses staged a play in the temple as Khapra watched sitting in a window of the divine edifice. He was reminded of Sidharao’s despondence and thought that this was the temple which would meet the king’s expectations. He found out from the, Gods that the same miracle would be enacted again on the night of the day after next and rushed back to Patana where he gave a graphic account to the king. The king came to the same spot and saw the temple which fully satisfied him. The Gods told him how to find the master architect who would build a similar temple for him. It took sixteen years to be completed, even though thousands of artisans were employed.

Nainasiri has included in his chapter on the Rudramala a poem written in its praise by Lalla Bhatta. The first two stanzas which describe the architecture and sculptures of the temple are as follows:

Fourteen storeys rise above the earth and seven thousand pillars,
In row after row, while eighteen hundred statues studded with emeralds adorn it.
It is endowed with thirty thousand flagstaffs with stems carved and leaves of gold.
Seven thousand sculptured elephants and horses stand in attendance on Rudra.
Seeing it all, Gods and men get struck with wonder and are greatly charmed,
Jayasimha has built a temple which excites the envy of emperors. The sculptured elephants and lions trumpet and roar, all around, again and again,
The golden kalasas glitter on the mandapa upheld by numerous pillars.
The statues sing and dance and roll their eyes,
So that even the Gods jump with joy and blow their conches.
The ecstatic dance of Gods is watched by Gods and men who crowd around,
That is why the Bull, O Sidha! O King of Kings! is feeling frightened.

The original from the book below.

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A modern expert on medieval Hindu architecture has speculated about the Rudramahalaya on the basis of what has survived. The Solanki tradition maintains, writes Dr. S.K. Saraswati, a rich and prolific output in the twelfth century AD which saw two eminent royal patrons of building art in Siddharaja Jayasimha and Kumarapala. With the former is associated the completion of an imposing conception, the Rudra Mala or Rudra Mahalaya, at Siddhapur (Gujarat). Unfortunately it is now completely in ruins but a picture of its former splendour seems to have survived in a Gujarati ballad which speaks of the temple as covered with gold, adorned with sixteen hundred columns, veiled by carved screens and pierced lattices, festooned with pearls, inlaid with gems over the doorways and glistening with rubies and diamonds. Much of this is, no doubt, exaggeration full of rhetoric; but the impressive character of the conception is evidenced by the scanty, though co-lossal, remains. They consist of groups of columns of the pillared maNDapa, which seems to have been in more than one storey, and had three enterance porticos on three sides. The surviving foundations suggest that the conception with the usual appurtenances occupied a space nearly 300 feet by 230 feet. In front there stood a kirti-torana of which one column still remains. From the dimensions the Rudra Mala seems to have been one of the largest architectural conceptions in this area. The rich character of its design is fully evident in the few fragments that remain.

The Jami Masjid

The Note from the Government of Gujarat says that the temple was destroyed and three shrines in the eastern flank of the temple were converted into a mosque but there is no evidence as to the date of conversion. The Report of the Minorities Commission gives more details about the destruction and conversion of the temple. This temple, says the Report, seems to have been destroyed partly by Ulugh Khan in AD 1297-98 and partly by Ahmadshah in AD 1415. Some of the cubicles and a number of pillars on the Western side of the temple, it would appear were later converted into a mosque. The prayer hall of the mosque so converted has three domes. In the Western (Qaba) waft of the mosque Mimbar and Mehrabs were provided by using the doors of the shrines which were then filled with debris. The exact date of conversion of this part of Rudramahalaya complex is not known. However, according to inscriptions at the entrance it appears that the mosque known as Jama Masjid, was constructed during the reign of Aurangzeb in 1645.

B.L. Nagarch, on the other hand, writes that the inscription fixed in the modern entrance gate to the mosque mentions the construction of shops by Ali Askari in Adil Ganj and there is no reference to the mosque. Moreover, Aurangzeb was not the ruling Mughal monarch in 1645, having ascended the throne thirteen years later in 1658. The temple remains discovered inside the mosque also go to show that at least that part of the structure was built not long after the Rudramahalaya was demolished. The Minorities Commission, it seems, has relied upon some local tradition about Aurangzeb having built the mosque. Aurangzeb did live in Gujarat in 1645 when he was appointed Governor of that province by Shah Jahan. He also destroyed Hindu temples in Gujarat as is evident from his firman dated November 20, 1665 which says that “In Ahmadabad and other parganas of Gujarat in the days before my accession (many) temples were destroyed by my order.” It seems that somewhere along the line several stories have got mixed up and Aurangzeb has been credited with a pious deed he did not perform at Sidhpur, not at least in respect of the Jami Masjid built on the site and from the debris of the Rudramahalaya. What might have happened is that some major repairs to the Jami Masjid were carried out while he was the Governor of Gujarat and at his behest. The subject needs examination with reference to records, if any.

Nor do we find a specific mention of Sidhpur or the Rudramahalaya in the available accounts of Ulugh Khan’s invasion of Gujarat. The Minoritiesð Commission has made a mistake in giving the date of the invasion as AD 1297-98. The correct date is 1299.

There is, however, no doubt that Ahmad Shah I (AD 1411- 43), the Sultan of Gujarat, destroyed the Rudramahalaya and raised a mosque on the site. Soon after his return to Ahmadabad, writes S.A.I. Tirmizi, Ahmad marched to Sidhpur, which was one of the most ancient pilgrim centres in north Gujarat. It was studded with beautiful temples, some of which were laid low. A.K. Majumdar is more specific. Ahmad Shah like his grandfather, he says, was a bigot and seized every opportunity to demolish Hindu temples. In 1414, he appointed one Taj-ul-Mulk to destroy all temples and to establish Muslim authority throughout Gujarat. According to Firishta, the task was executed with such diligence that the names of Mawass and Girass (i.e. Hindu zamindars) were hereafter unheard of in the whole kingdom. Next year Ahmad attacked the celebrated city of Sidhpur in north Gujarat where he broke the images in the famous Rudramahalaya temple and converted it into a mosque.

A poetic account of what Ahmad Shah did at Sidhpur is available in Mirat-i-Sikandari, the history of Gujarat, written by Sikandar ibn-i-Muhammad alias Manjhu ibn-i-Akbar in the first quarter of the sixteenth century. He marched on Saiyidpur, writes the historian, on Jamad-ul-Awwal in AH 818 (July/August, AD 1415) in order to destroy the temples which housed idols of gold and silver.

He marched under divine inspiration,
For the destruction of temples at Saiyidpur,
Which was a home of the infidels,
And the native place of accursed fire-worshippers.
There they dwelt, day and night,
The thread-wearing idolaters.
It had always remained a place for idols and idol-worshippers,
It had received no injury whatsoever from any quarter.
It was a populous place, well-known in the world,
This native place of the accursed infidels.
Its foundations were laid firmly in stone,
It was decorated with designs as if drawn from high heaven.
It had doors made of sandal and ud.
It was studded with rings of gold,
Its floors were laid with marble,
Which shone like mirrors.
Ud was burnt in it like fuel,
Candles of camphor in large numbers were lighted in it.
It had arches in every corner,
And every arch had golden chandeliers hanging in it.
There were idols of silver set up inside,
Which put to shame the idols of China and Khotan.
Such was this famous ancient temple,
It was famous all over the world.
By the effort of Ahmad, it was freed from the idols,
The hearts of idol-worshippers were shattered with grief.
He got mosques constructed, and mimbars placed in them,
From where the Law of Muhammad came into force.
In place of idols, idol-makers and idol-worshippers,
Imams and callers to prayers and khatibs were appointed.
Ahmad’s good grace rendered such help,
That an idol-house became an abode of Allah.