Because the bridge was wooden and was deteriorating, it was posted at a weight limit of 3 tons in 2001. The bridge was severely damaged by the Mother’s Day Floods of May 2006, in which Beaver Brook completely washed away the culvert and caused the wooden superstructure to rot and partially give way. The bridge was closed indefinitely the next month after it was determined that an immediate repair was not feasible. The bridge remained closed for over a decade, until it finally collapsed under the weight of the February Nor’Easter in 2017.
After a long series of negotiations, the town of Windham in conjunction with NHDOT have decided that they are not going to replace the Castle Hill Road Bridge, and therefore the Mammoth Road access to the neighborhoods within will be closed indefinitely.
The primary reason for this is that residents and people affiliated with Windham High School, which is located on the NH-111 side of the road (the road is known as London Bridge Road on that end) were concerned that replacing the bridge and reopening the road would result in a significant increase in traffic using the road as a shortcut/bypass and would therefore end the area’s ability to be a quiet residential neighborhood. There has been a significant increase in the amount of traffic using both Mammoth Road and NH-111 since the closure of the Castle Hill Bridge in 2007, and therefore residents on both ends are concerned about the increase in traffic a reopened bridge could bring.
Reportedly, an historical marker to indicate the location of this bridge prior to collapse is going to be erected, although I have not heard anything more than that.
I visited the bridge site today and discovered an interesting situation. The original bridge has been replaced with a new modern concrete slab bridge, but yet was still posted for a 10 ton weight limit. However, the bridge is still completely closed to vehicular traffic... which makes the weight limit posting strange. I am unsure when this replacement bridge was put into place, but as far as I know the road was never reopened to through traffic. I find it curious that they would bother putting in a replacement bridge if they indeed were never planning to reopen the road as mentioned by Brianna below (which seem very reasonable... Windham High School aside, the vast majority of the houses in the area appear to be extremely wealthy and high-value developments). You can walk out on the new bridge, although there is technically no "legal" place to park (I parked directly in front of the Jersey barrier on the south side, which worked out okay. The north side is too narrow and is right next to an active intersection). The views of Beaver Brook are nice, but the new bridge is definitely inferior to the original wooden bridge/culvert in more ways than one.