Altun Ha is the most visited Mayan site in Belize as it is easily accessible from Belize City. Groups A and B form the central precinct of Altun Ha. Together with groups C, D, and E they form the urban area and zones G-N the suburbs. The city even had two causeways. One of them was likely a raised walkway through a swampy area that may have been impassable without it.
The oldest structures are located in zone C, which you cannot visit.
More details can be found here: https://backpackandsnorkel.com/Belize/Altun-Ha/