Baitul or Baytul-Maqdis is itself the Masjid Al-Aqsa mosque. Baytul-Maqdis, the last of the three sacred sites, other being Masjid Al Haram and Masjd An Nabwi.
About which Allah (Glory be to Him) Says (what means): Exalted is He who took His Slave [i.e. Prophet Muhammadﷺby night from al-Masjid al-Haram to al-Masjid al-Aqsa, whose surroundings We have blessed, to show him of Our signs. Indeed, He is the Hearing, the Seeing. Quran.Surah Isra' 17:1 
It was built by Sulaymaan (peace be upon him), as stated in the hadith of Sunan al-Nasaa’i. Ten years after the Prophet Mohammad (pbuh) received his first revelation, he made a miraculous night journey from Mecca to Jerusalem and to the Seven Heavens on a white flying horse called Al-Buraq. Today, Muslims throughout the World use Mecca as the direction of prayers (Qibla). However, for 16½ months following the Prophet Mohammad's miraculous journey, Jerusalem was the Qibla. 
The rectangular Al-Aqsa Mosque is 144,000 square meters, 35 acres, or 1/6 of the entire area within the walls of the Old City of Jerusalem as it stands today. It is also called Al-Haram El-Sharif (the Nobel Sanctuary).
Al-Aqsa Mosque holds up to 400,000 worshippers at one time, bearing in mind that the space required for each person is roughly 0.8m x 0.5m to enable the submissive kneeling in prayer. On Fridays at noon, during the fasting month of Ramadan, and particularly the 27th of Ramadan (Lailat El-Qadr), the area is filled to virtual capacity.
There are 11gates to Al-Aqsa Mosque: 7 of which are open. Of the 4 closed gates, one is the Golden Gate.
It was narrated from 'Abdullah bin 'Amr that the Messenger of Allah (ﷺ) said:"When Sulaiman bin Dawud finished building Bait Al-Maqdis, he asked Allah for three things: Judgement that was in harmony with His judgement, and he was given that. And he asked Allah for a dominion that no one after him would have, and he was given that. And when he finished building the Masjid he asked Allah, the Mighty and Sublime, that no one should come to it, intending only to pray there, but he would emerge free of sin as the day his mother bore him." Classed as Sahih by Shaikh Albani in Sunan an-Nasa'i 693
Al-Masjid al-Aqsa (in Jerusalem) was the first of the two Qiblahs(Direction of offering Salah), and is one of the three mosques to which people may travel for the purpose of worship. The Prophetﷺsaid: “Do not travel or set out on journey(specifically) to any mosque except three: al-Masjid al-Haraam, Masjid al-Aqsa, and this mosque of mine.” Sahih al-Bukhaari, Vol.2: 1189, Vol.3: 1996 and Sahih Muslim 1397
Abu Dharr (may Allaah be pleased with him) who said: “I said, ‘O Messenger of Allaah, which mosque was built on earth first?’ He said, ‘Al-Masjid al-Haraam [in Makkah].’ I said, ‘Then which?’ He said, ‘Al-Masjid al-Aqsa.’ I said, ‘How much time was there between them?’ He said, ‘Forty years. So wherever you are when the time for prayer comes, pray, for that is the best thing to do.’” Sahih Al Bukhari, Vol.4: 3425, Sahih Muslim, 520 and Sunan Ibn Majah 753
The Prophetﷺwas taken on the Night Journey (isra’) to Bayt al-Maqdis (Jerusalem), where he led the Prophets in prayer in this blessed mosque.
Allaah says: “Glorified (and Exalted) be He (Allaah) [above all that (evil) they associate with Him] Who took His slave (Muhammad) for a journey by night from Al-Masjid Al-Haraam (at Makkah) to Al-Masjid Al-Aqsaa (in Jerusalem), the neighbourhood whereof We have blessed, in order that We might show him (Muhammad) of Our Ayaat (proofs, evidences, lessons, signs, etc.). Verily, He is the All-Hearer, the All-Seer” Quran Surah al-Isra’ 17:1 
One prayer in al-Masjid al-Aqsa is equivalent to two hundred and fifty prayers offered elsewhere.
It was narrated that Abu Dharr (may Allaah be pleased with him) said: We were discussing when we were with the Messenger of Allaahﷺ, which is better, the Mosque of the Messenger of Allaah (peace and blessings of Allaah be upon him) or Bayt al-Maqdis (Jerusalem). The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “One prayer in my mosque is better than four prayers offered there (in Bayt al-Maqdis), and what a good place of prayer it is. Soon there will come a time when, if a man has a piece of land the size of a horse’s rope from which he can see Bayt al-Maqdis, that will be better for him than the whole world.” Mustadrak al-Haakim, 4/509; he classed it as saheeh and al-Dhahabi and al-Albaani agreed with him, as it says in al-Silsilah al-Saheehah, at the end of the discussion on hadeeth no. 2902.
One prayer offered in the Prophet’s Mosque is equivalent to one thousand prayers (offered elsewhere), so one prayer offered in al-Masjid al-Aqsa is equivalent to two hundred and fifty prayers.
With regard to the well-known hadeeth that says that one prayer offered there is equivalent to five hundred prayers, this hadeeth is da’eef (weak). Tamaam al-Minnah by Shaykh al-Albaani (may Allaah have mercy on him), p. 292. 
Allah knows best