Normally visitors to Cambodia write in this tip section about the mines...but I was assured that the whole area was cleared by 2003 so I was more worried about the insects!
The Temple is surrounded by a moat which was only muddy at the start of the raining season, but nevertheless a great attraction for mosquitos. Don't know if there are any dangerous types but anyway I got bitten at least 30 times in 10 minutes, right through my polo shirt, many of the bites close toghether causing huge red bumps on my upper body.
Fortunately the inner complex was mosquito free.
Also I encountered some huge spiders in their webs, I tried to be careful not to hit those as a mosquito is somehow to cope with but what about an angry 20 cm long spider crawling on your chest?!
Many types of snakes are living around the Angkor sites and although not very often sighted around Siem Riep due to large crowds, in places like Beng Mealea I recommend to be more careful.
I spoke a local farmer from the village working in the fields around Beng Mealea. Based on first hand accounts he said there are 3 common types of snakes around but no people die from the bites. Instead they cause severe pains for days to weeks and likely could destroy the rest of your holiday.
If you want to do some decent exploring here, for footwear I would recommend closed shoes.
As you wander often through tall grass and bushes you can't always see what you step on. Also there's a lot of scrambling on stones available here so slippers might not be the right choice anyway...
There are several places at Beng Mealea that are essentially underground and not lighted. Because there was little light, the pictures came out fuzzy. In one place a mysterious energy tried to take over my body. Hey, look at the pictures!