I found a good way to beat Cells was to build collectors up to the openings early. Then build mortars within range of the pools, this will give you time to build up your collectors. As you build forward, use blasters to get within reach by pushing back slowly. Remember that you can safely build on the sides between chambers.
On the subject of spelling, "defense" is the recognized American English spelling. "Defence" is the recognized British English spelling. Really comes down to personal preference, and I think I vary at times.
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