Hi and welcome!
I think this should be the closest field to you that I know off... but I'm not sure if speedball is possible?
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http://www.paintballarena.be
Either way, friends of mine are opening an indoor field in Gent, pretty central in Belgium... be sure to drop by... it should be opening for business in the next few weeks... it's almost finished... they have 2 spots reserved for sup air, but they still have to be delivered... in the meanwhile there is already a nice rec field.
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info@paintballgent.be